Fix 3xhead_flutter_hooks.sh script not being executable.

Change-Id: If4fc09d64afd80881f10875f77f8aeca9d05fb2f
Reviewed-on: https://dart-review.googlesource.com/69963
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Jonas Termansen <sortie@google.com>
diff --git a/tools/3xhead_flutter_hooks.sh b/tools/3xhead_flutter_hooks.sh
old mode 100644
new mode 100755