package/python-pillow: add webpmux support

webpmux is an optional dependency since version 2.2.0 and
https://github.com/python-pillow/Pillow/commit/b4735f7829bb88c99071cd91b208aa6ffd2cba24



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: move into existing webp conditional block]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>