package/qt5multimedia: fix build without opengl

Fixes:
 - http://autobuild.buildroot.org/results/fc7bcc6b3ccf1e075dce43296b36e7e1cac417db



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>