qt: Don't define GLsync inside gstglfuncs.h

qt: Don't define GLsync inside gstglfuncs.h

This was originally added for fixing conflicting definitions between
Android and Qt, but times have changed and now this breaks the build
on iOS:

 [...]/OpenGLES.framework/Headers/ES3/gl.h:1006:48: error: unknown type name 'gst_qt_GLsync'
 GL_API void           GL_APIENTRY glGetSynciv (GLsync sync, GLenum pname, GLsizei bufSize, GLsizei* length, GLint* values)   OPENGLES_DEPRECATED(ios(7.0, 12.0), tvos(9.0, 12.0));
                                               ^
 ../ext/qt/gstqtgl.h:49:16: note: expanded from macro 'GLsync'
 #define GLsync gst_qt_GLsync
                ^
 6 errors generated.

Instead, we simply avoid defining GLsync ourselves if we're using Qt.