[PATCH/libiberty] Add missing <sys/types.h> in unlink-if-ordinary.c

[DJ Delorie]

> <sys/types.h> is not an ISO C header file.  But it is in POSIX.1.
> And it is available on every Unix system.  Because of that, it is
> available on every Unix workalike system.

Right, but the machines that wouldn't have it, probably would have K&R
compilers anyway.  We haven't had a problem using it unprotected so
far, and there's even less reason to worry about it now.