platforms/x11: Build own vsync monitors only when GLX backend is built

The SGIVideoSyncVsyncMonitor and OMLSyncControlVsyncMonitor use GLX,
thus they should be compiled only if the GLX backend is built.


(cherry picked from commit 6aea213c)

plugins/platforms/x11/standalone/CMakeLists.txt

@@ -5,10 +5,8 @@ set(X11PLATFORM_SOURCES
     eglbackend.cpp
     logging.cpp
     non_composited_outline.cpp
-    omlsynccontrolvsyncmonitor.cpp
     overlaywindow_x11.cpp
     screenedges_filter.cpp
-    sgivideosyncvsyncmonitor.cpp
     windowselector.cpp
     x11_decoration_renderer.cpp
     x11_output.cpp
@@ -31,6 +29,8 @@ if (HAVE_EPOXY_GLX)
         glx_context_attribute_builder.cpp
         glxbackend.cpp
         glxconvenience.cpp
+        omlsynccontrolvsyncmonitor.cpp
+        sgivideosyncvsyncmonitor.cpp
     )
 endif()