The following Cflags has been added to libmfx.pc, so mfx/ prefix is no
longer needed when including mfx headers in FFmpeg.
   Cflags: -I${includedir} -I${includedir}/mfx

Some old versions of libmfx have the following Cflags in libmfx.pc
   Cflags: -I${includedir}

We may add -I${includedir}/mfx to CFLAGS when running 'configure
--enable-libmfx' for old versions of libmfx, if so, mfx headers without
mfx/ prefix can be included too.

If libmfx comes without pkg-config support, we may do a small change to
the settings of the environment(e.g. set -I/opt/intel/mediasdk/include/mfx
instead of -I/opt/intel/mediasdk/include to CFLAGS), then the build can
find the mfx headers without mfx/ prefix

After applying this change, we won't need to change #include for mfx
headers when mfx headers are installed under a new directory.

This is in preparation for oneVPL support (mfx headers in oneVPL are
installed under vpl directory)