Add Xlib for eglconvenience

This used to work by accident but now with the recent build system changes
the problem became apparent. Not listing the X11 libs is wrong anyway.

Change-Id: I6f75dafa81510d6d6a7571a9fe156cc7b968c8dc
Reviewed-by: Andy Nichols <andy.nichols@theqtcompany.com>

src/platformsupport/eglconvenience/eglconvenience.pri

@@ -23,6 +23,7 @@ contains(QT_CONFIG,egl) {
             $$PWD/qxlibeglintegration_p.h
         SOURCES += \
             $$PWD/qxlibeglintegration.cpp
+        LIBS_PRIVATE += $$QMAKE_LIBS_X11
     }
     CONFIG += egl
 }