export isl_pw_multi_aff_zero
export isl_pw_multi_aff_zero

This complements the already exported isl_multi_*_zero and
likewise needs to be exported as a named constructor.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl