fix ocaml on xen-4.5-rc4 build

Building xen-4.5-rc4 on Deb8
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// fix to get ocaml to build in the normal dir
export OCAMLDESTDIR=$(pwd)/dist/install/usr/lib/ocaml/

make dist-tools
export XEN_TARGET_ARCH=x86_64
fakeroot ./tools/misc/mkdeb $(pwd) 4.5.0-rc4

dpkg-deb -c ./dist/xen-upstream-4.5.0-rc4.deb | grep ocaml | awk '{print $6}'
./etc/ld.so.conf.d/ocaml.conf
./usr/lib/ocaml/
./usr/lib/ocaml/xenlight/
./usr/lib/ocaml/xenlight/libxenlight_stubs.a
./usr/lib/ocaml/xenlight/META
./usr/lib/ocaml/xenlight/xenlight.cma
./usr/lib/ocaml/xenlight/xenlight.cmxa
./usr/lib/ocaml/xenlight/xenlight.a
./usr/lib/ocaml/xenlight/xenlight.cmx
./usr/lib/ocaml/xenlight/xenlight.cmi
./usr/lib/ocaml/xenctrl/
./usr/lib/ocaml/xenctrl/xenctrl.cmx
./usr/lib/ocaml/xenctrl/xenctrl.cma
./usr/lib/ocaml/xenctrl/META
./usr/lib/ocaml/xenctrl/xenctrl.a
./usr/lib/ocaml/xenctrl/libxenctrl_stubs.a
./usr/lib/ocaml/xenctrl/xenctrl.cmi
./usr/lib/ocaml/xenctrl/xenctrl.cmxa
./usr/lib/ocaml/xenmmap/
./usr/lib/ocaml/xenmmap/xenmmap.cmx
./usr/lib/ocaml/xenmmap/xenmmap.cma
./usr/lib/ocaml/xenmmap/META
./usr/lib/ocaml/xenmmap/xenmmap.cmi
./usr/lib/ocaml/xenmmap/xenmmap.cmxa
./usr/lib/ocaml/xenmmap/xenmmap.a
./usr/lib/ocaml/xenmmap/libxenmmap_stubs.a
./usr/lib/ocaml/xentoollog/
./usr/lib/ocaml/xentoollog/libxentoollog_stubs.a
./usr/lib/ocaml/xentoollog/META
./usr/lib/ocaml/xentoollog/xentoollog.cmx
./usr/lib/ocaml/xentoollog/xentoollog.cma
./usr/lib/ocaml/xentoollog/xentoollog.cmi
./usr/lib/ocaml/xentoollog/xentoollog.a
./usr/lib/ocaml/xentoollog/xentoollog.cmxa
./usr/lib/ocaml/xenstore/
./usr/lib/ocaml/xenstore/xenstore.cmx
./usr/lib/ocaml/xenstore/META
./usr/lib/ocaml/xenstore/xenstore.cmxa
./usr/lib/ocaml/xenstore/xenstore.a
./usr/lib/ocaml/xenstore/xenstore.cmi
./usr/lib/ocaml/xenstore/xenstore.cma
./usr/lib/ocaml/xenstore/xenstore.cmo
./usr/lib/ocaml/stublibs/
./usr/lib/ocaml/stublibs/dllxenctrl_stubs.so
./usr/lib/ocaml/stublibs/dllxenbus_stubs.so
./usr/lib/ocaml/stublibs/dllxenmmap_stubs.so.owner
./usr/lib/ocaml/stublibs/dllxenmmap_stubs.so
./usr/lib/ocaml/stublibs/dllxeneventchn_stubs.so.owner
./usr/lib/ocaml/stublibs/dllxenlight_stubs.so.owner
./usr/lib/ocaml/stublibs/dllxentoollog_stubs.so
./usr/lib/ocaml/stublibs/dllxenbus_stubs.so.owner
./usr/lib/ocaml/stublibs/dllxenlight_stubs.so
./usr/lib/ocaml/stublibs/dllxenctrl_stubs.so.owner
./usr/lib/ocaml/stublibs/dllxentoollog_stubs.so.owner
./usr/lib/ocaml/stublibs/dllxeneventchn_stubs.so
./usr/lib/ocaml/xenbus/
./usr/lib/ocaml/xenbus/META
./usr/lib/ocaml/xenbus/xenbus.cmxa
./usr/lib/ocaml/xenbus/xenbus.cmx
./usr/lib/ocaml/xenbus/xenbus.a
./usr/lib/ocaml/xenbus/xenbus.cma
./usr/lib/ocaml/xenbus/libxenbus_stubs.a
./usr/lib/ocaml/xenbus/xenbus.cmo
./usr/lib/ocaml/xenbus/xenbus.cmi
./usr/lib/ocaml/xeneventchn/
./usr/lib/ocaml/xeneventchn/META
./usr/lib/ocaml/xeneventchn/xeneventchn.cmi
./usr/lib/ocaml/xeneventchn/xeneventchn.a
./usr/lib/ocaml/xeneventchn/xeneventchn.cmx
./usr/lib/ocaml/xeneventchn/xeneventchn.cmxa
./usr/lib/ocaml/xeneventchn/libxeneventchn_stubs.a
./usr/lib/ocaml/xeneventchn/xeneventchn.cma