isl_pw_*_eval: fix type of variable
isl_pw_*_eval: fix type of variable

This was missing from isl-0.14.1-479-g370a8b7d07 (introduce isl_bool and
isl_stat return types, Mon May 4 09:45:16 2015 +0200).

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

PollyLabs/isl,Meinersbur/isl,PollyLabs/isl,PollyLabs/isl,Meinersbur/isl,inducer/isl-mirror,Meinersbur/isl,inducer/isl-mirror,PollyLabs/isl,inducer/isl-mirror,inducer/isl-mirror,Meinersbur/isl,inducer/isl-mirror,PollyLabs/isl