package/python-matplotlib: add QT5 support

python-matplotlib may be used in qt5 application. This patch add option to
enable qt5. Since this backend is automatically enabled when pyqt5 is detected,
list dependencies is simply an updated (no enable/disable options).

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>