libctf: avoid strndup

libctf: avoid strndup

Not all platforms have it.  Use libiberty xstrndup() instead.

(The include of libiberty.h happens in an unusual place due to the
requirements of synchronization of most source files between this
project and another that does not use libiberty.  It serves to pull
libiberty.h in for all source files in libctf/, which does the trick.)

Tested on x86_64-pc-linux-gnu, x86_64-unknown-freebsd12.0,
sparc-sun-solaris2.11, i686-pc-cygwin, i686-w64-mingw32.

libctf/
	* ctf-decls.h: Include <libiberty.h>.
	* ctf-lookup.c (ctf_lookup_by_name): Call xstrndup(), not strndup().