Avoid c-ctype.h in libinproctrace.so

libinproctrace.so doesn't link against gnulib, and some versions of
clang won't inline the c-ctype.h functions.  This causes link
failures.

This patch works around the problem by reverting to <ctype.h> in this
case.

Tested-By: Guinevere Larsen <guinevere@redhat.com>