python-pyqt: fix opengl build failure

Fix:
http://autobuild.buildroot.net/results/206/2062208c171207428c9121215971e00c52bf306a/



According to configure-ng.py, PyQt_NoOpenGLES must be added to qtdetail when
QT_OPENGL_ES is set.

[Thomas: add comment to explain the double negation.]

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>