Qt: Add GLES defines if USING_GLES2.

See #11495 - thanks to Jan Beich.

CMakeLists.txt

@@ -784,6 +784,9 @@ elseif(USING_QT_UI)
       Qt/mainwindow.h
     )
     add_definitions(-DUSING_QT_UI)
+    if(USING_GLES2)
+        add_definitions(-DQT_OPENGL_ES -DQT_OPENGL_ES_2)
+    endif()
     include_directories(${CMAKE_CURRENT_BINARY_DIR} Qt)
 	set(nativeExtraLibs ${nativeExtraLibs} Qt5::OpenGL Qt5::Gui Qt5::Core)
 	set(TargetBin PPSSPPQt)