gpu.c: get_non_parallel_subtree_filters: use isl_bool for local variable
gpu.c: get_non_parallel_subtree_filters: use isl_bool for local variable

This was missing from ppcg-0.07-21-g56f199d9 (gpu.c:
has_any_permutable_node: return isl_bool, Thu Sep 14 17:26:13 2017 +0200).

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

Meinersbur/ppcg,Meinersbur/ppcg