[FIX] base: support flex-wrap in wkhtmltopdf

As explained here:
https://github.com/odoo/odoo/commit/770aec399c4e6e29cfc5854318cbda19a605315d#diff-3d7991d8bc534e0372f0bd9b4f9165a7R424


we have to post-process the generated css to add some vendor prefixes.

The flex-wrap property was not post-processed yet.

Related to opw-2045424

closes odoo/odoo#36182

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>