Compile when EGL headers are located in non-standard location

Task-number: QTBUG-30101
Change-Id: I0fb774392c3d4fc7551ba31d1546788c78ca8afc
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/platformsupport/eglconvenience/eglconvenience.pri

@@ -12,5 +12,6 @@ contains(QT_CONFIG,egl) {
         SOURCES += \
             $$PWD/qxlibeglintegration.cpp
     }
+    CONFIG += egl
 }