qt: fix build warning with clang and c-linkage of user defined type
In file included from ../subprojects/gst-plugins-good/ext/qt/qtglrenderer.cc:17:
../subprojects/gst-plugins-good/ext/qt/gstqtglutility.h:35:16: error: 'qt_opengl_native_context_from_gst_gl_context' has C-linkage specified, but returns user-defined type 'QVariant' which is incompatible with C [-Werror,-Wreturn-type-c-linkage]
QVariant qt_opengl_native_context_from_gst_gl_context (GstGLContext * context);