Check for pkg-config.

The configure script uses pkg-config unconditionally, so you need it if
you're going to build Wireshark with the autotools.

Change-Id: Ibb0e5e37651256c7134303c8176f30a9150af699
Reviewed-on: https://code.wireshark.org/review/6085
Reviewed-by: Guy Harris <guy@alum.mit.edu>