[Polly-ACC] Add isl_space.h to gpu_tree.c

[Polly-ACC] Add isl_space.h to gpu_tree.c

Summary:
This patch adds <isl_space.h> to gpu_tree.c. This prevents a segfault
when allocating a new isl_space in the function create_sync_domain(), as
the compiler now knows that the return type is a pointer instead of
assuming the function returns an int.

This has been updated in upstream PPCG, so we should bump up our PPCG
version.

Initially discovered by Philip Pfaffe in Polly.

Reviewers: grosser, bollu, philip.pfaffe

Reviewed By: bollu

Subscribers: nemanjai, kbarton, llvm-commits

Differential Revision: https://reviews.llvm.org/D48883

Contributed-by: Alain Denzler <alaindenzler@gmail.com>
llvm-svn: 336251