package/qt5multimedia: fix build with gstreamer 0.10

Fixes:
 - http://autobuild.buildroot.org/results/de26fd4f5ab130bd87ea9d7709d9568290362758



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
[Arnout: patch applies to 5.12.2 version only]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>