Qt5 cmake: Fix the case where a single library appears in multiple _qt5gui_find_extra_libs calls

Necessary on the raspberry pi, because Broadcom's EGL needs libEGL and libGLESv2, but OpenGL ES 2 is libGLESv2 only.
Should work on all other platforms, too. Will file an upstream bug once we've solved any remaining issues. Hopefully this is last :)
Fixes the stellarium build.