The Nim:

import os, strutils
import nimterop/[cimport, build, paths]

cPlugin:
    proc onSymbol*(sym: var Symbol) {.exportc, dynlib.} =
        sym.name = sym.name.strip(chars = {'_'})

cImport("/usr/include/fitsio.h", recurse=true)

The error:

/home/liam/.cache/nim/nimterop/cPlugins/nimterop_962481396.nim(5, 22) Error: attempting to call undeclared routine: 'strip'