Disable QtMultimedia on Linux for now

Apparently there are build problems when it's enabled:

[2019-03-11T15:37:22.559Z] format.setCodec("audio/pcm");
[2019-03-11T15:37:22.559Z] ^
[2019-03-11T15:37:22.559Z] qgstutils.cpp: In function ���GstCaps* QGstUtils::videoFilterCaps()���:
[2019-03-11T15:37:22.559Z] qgstutils.cpp:1326:5: error: braces around scalar initializer for type ���GstCapsFlags���
[2019-03-11T15:37:22.559Z] };
[2019-03-11T15:37:22.559Z] ^
[2019-03-11T15:37:22.559Z] make[7]: * [.obj/qgstutils.o] Error 1
[2019-03-11T15:37:22.559Z] make[6]: * [sub-gsttools-make_first] Error 2
[2019-03-11T15:37:22.559Z] make[5]: * [sub-src-make_first] Error 2
[2019-03-11T15:37:22.559Z] make[4]: * [module-qtmultimedia-make_first] Error 2
[2019-03-11T15:37:22.559Z] make[3]: * [ext_qt/ext_qt-prefix/src/ext_qt-stamp/ext_qt-build] Error 2
[2019-03-11T15:37:22.559Z] make[2]: * [ext_qt/CMakeFiles/ext_qt.dir/all] Error 2
[2019-03-11T15:37:22.559Z] make[1]: * [ext_qt/CMakeFiles/ext_qt.dir/rule] Error 2
[2019-03-11T15:37:22.559Z] make: * [ext_qt] Error 2