Fix the CMake WinPcap library path check.

Look for wpcap.lib in WpdPack/Lib/x64 when our target platform is Win64.

Change-Id: I9a1bac22106bcb6a1f155ce83c02e344e6ec0d55
Reviewed-on: https://code.wireshark.org/review/3331
Reviewed-by: Gerald Combs <gerald@wireshark.org>