openfst: Use allocator_traits::allocate in memory.h

--- src/include/fst/memory.h
+++ src/include/fst/memory.h
@@ -241,7 +241,8 @@ class BlockAllocator {
     if (n * kAllocFit <= kAllocSize) {
       return static_cast<T *>(Arena()->Allocate(n));
     } else {
-      return Allocator().allocate(n, hint);
+      auto alloc = Allocator();
+      return std::allocator_traits<Allocator>::allocate(alloc, n, hint);
     }
   }

@@ -307,7 +308,8 @@ class PoolAllocator {
     } else if (n <= 64) {
       return static_cast<T *>(Pool<64>()->Allocate());
     } else {
-      return Allocator().allocate(n, hint);
+      auto alloc = Allocator();
+      return std::allocator_traits<Allocator>::allocate(alloc, n, hint);
     }
   }