[FIX] requirements: add back requirement

Oversight of e554ce34

closes odoo/odoo#33322

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

requirements.txt

@@ -10,6 +10,7 @@ greenlet==0.4.10 ; python_version < '3.7'
 greenlet==0.4.13 ; python_version >= '3.7'
 html2text==2016.9.19
 Jinja2==2.10.1
+libsass==0.12.3
 lxml==3.7.1 ; sys_platform != 'win32' and python_version < '3.7'
 lxml==4.2.3 ; sys_platform != 'win32' and python_version >= '3.7'
 lxml ; sys_platform == 'win32'