build: Tweak CMakeLists.txt pkg-config usage for finding avutil.

* CMakeLists.txt: Prepend CMAKE_PREFIX_PATH with the location of
libavutil.pc from daemon's contrib before trying to find libavutil
using pkg-config, and remove it afterwards.  Further, there is no need
to link the application executable against libavutil; the application
only needs its headers (libclient, however, does already link against
libavutil).

Change-Id: Iffa5af43bfd1bb62bc24cc94dce292f87e9ecb2a