Compile Fix: unused variable in q_refreshRateFromFb() on non-linux

To fix a compile warning under non-Linux systems, declare
framebufferDevice in q_refreshRateFromFb() as Q_UNUSED like in the other
functions.

Change-Id: I31a4e788f07d27d3cff7f6ea7fd82813a7acc7d9
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/platformsupport/eglconvenience/qeglconvenience.cpp

@@ -578,6 +578,10 @@ int q_screenDepthFromFb(int framebufferDevice)
 
 qreal q_refreshRateFromFb(int framebufferDevice)
 {
+#ifndef Q_OS_LINUX
+    Q_UNUSED(framebufferDevice)
+#endif
+
     static qreal rate = 0;
 
 #ifdef Q_OS_LINUX