Fix warning

qxcbnativeinterface.cpp:209:8: warning: unused parameter ‘context’
[-Wunused-parameter]

Change-Id: I0c90e94020907cf84626af9f3fef3e6746182431
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/plugins/platforms/xcb/qxcbnativeinterface.cpp

@@ -231,6 +231,7 @@ void * QXcbNativeInterface::eglContextForContext(QOpenGLContext *context)
     return 0;
 #endif
 #else
+    Q_UNUSED(context)
     return 0;
 #endif
 }