Don't use extra variables for ws_mempbrk_sse42.c and popcount.c

Instead, just add to WSUTIL_FILES as necessary.

Change-Id: Iecadbd9a66ec54ee5d90aecfbfe5e636ae56e27e
Reviewed-on: https://code.wireshark.org/review/6043
Reviewed-by: Guy Harris <guy@alum.mit.edu>