Fix build on Gentoo with recent GStreamer 1.0.
Fix build on Gentoo with recent GStreamer 1.0.

Partially fixed by RR 125690, this also fixes for Gentoo in my testing,
and should for other distros where gstreamer places its gst-config under
gstreamer-1.0/include.

REVIEW:126554

KDE/phonon-gstreamer,KDE/phonon-gstreamer