QEglConfigChooser: Silence warning about fallthrough

Add Q_FALLTHROUGH, fixing:
qeglconvenience.cpp:267:9: warning: this statement may fall through [-Wimplicit-fallthrough=]

Change-Id: I764a5821f98982bc94ce5dc6a4efa81a431fd369
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/platformsupport/eglconvenience/qeglconvenience.cpp

@@ -268,7 +268,7 @@ EGLConfig QEglConfigChooser::chooseConfig()
             configureAttributes.append(EGL_OPENGL_ES_BIT);
             break;
         }
-        // fall through
+        Q_FALLTHROUGH();
     default:
         needsES2Plus = true;
         break;