xcb: fix unused static warning

The variable ptrXcursorLibraryLoadCursor is only used in code
wrapped in XCB_USE_XLIB.

Change-Id: If0b4997ac6e80a3709263d9793d1ca0dfd5907f3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/plugins/platforms/xcb/qxcbcursor.cpp

@@ -54,7 +54,9 @@
 QT_BEGIN_NAMESPACE
 
 typedef int (*PtrXcursorLibraryLoadCursor)(void *, const char *);
+#ifdef XCB_USE_XLIB
 static PtrXcursorLibraryLoadCursor ptrXcursorLibraryLoadCursor = 0;
+#endif
 static xcb_font_t cursorFont = 0;
 static int cursorCount = 0;