isl_ast_build_expr.c: is_stride_constraint: return isl_bool
isl_ast_build_expr.c: is_stride_constraint: return isl_bool

This clarifies what the possible return values are.

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

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