LSB requires an explicit header include

Makes the plugin build in the LSB environment

Change-Id: Ic9a54b984f7fc1f393853ec01e32886f49a041c6
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/plugins/platforms/xlib/qxlibkeyboard.cpp

@@ -48,6 +48,10 @@
 
 #include <QtCore/QTextCodec>
 
+#if defined(QT_LINUXBASE)
+#  include <X11/keysymdef.h>
+#endif
+
 #ifndef XK_ISO_Left_Tab
 #define XK_ISO_Left_Tab         0xFE20
 #endif