Make the new OpenGL function headers compile on their own

You can't use #ifndef QT_NO_OPENGL before including whatever is supposed
to define that. That's qconfig.h, included by qglobal.h.

Also, make sure that the desktop OpenGL code isn't activated when
compiling in OpenGL ES 2 mode -- QOpenGLFunctions_1_0_CoreBackend and
other classes aren't defined.

Change-Id: I127edf56d42257580579789d0566b7e11c21133b
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>