gst-plugins-good: Fix Qt detection on non-Linux

By forcing qt_method as qmake on non-Linux

Don't want to find Qt using pkg-config on non-Linux because:

1. It's actually not recommended by Qt
2. It causes gstqmlgl.pc to contain references to those pkgconfig files, but on non-Linux, you're using the official Qt binaries which require --define-prefix with pkg-config, which breaks static linking of gstreamer plugins, so we don't want that
3. --define-prefix makes cerbero's relocatable pkgconfig files completely useless

This is a regression from !1049 (merged)

Requires gstreamer!3628 (merged)