gstreamer1.0-plugins-bad: fix incorrect handling of Cflags in gstreamer-gl.pc file

gstreamer1.0-plugins-bad: fix incorrect handling of Cflags in gstreamer-gl.pc file

Cflags had GL_CFLAGS appended, which contained full sysroot path to libdrm include;
generally such dependencies should be handled by pkgconfig's Requires facility.
It's unlikely that this .pc file is used by anything, but it was causing a QA error.

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>