Fix khr autotest to work with non-standard locations

Just like the egl autotest in qtbase does, the paths have to be added from the
respective QMAKE variables, otherwise the test will never pass on systems where
EGL and friends are installed in a non-standard location.

Change-Id: Idaa2136b5574f6f320abb8a7074350b5838b402e
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>

tools/qmake/config.tests/khr/khr.pro

+!isEmpty(QMAKE_INCDIR_EGL): INCLUDEPATH += $$QMAKE_INCDIR_EGL
+!isEmpty(QMAKE_INCDIR_OPENGL): INCLUDEPATH += $$QMAKE_INCDIR_OPENGL
+
 CONFIG-=qt
+
 linux:SOURCES += khr.cpp