Fix warning with newer gcc in str_numa_mpol_cb()

GCC complains about potentially accessing the array beyond
the end. Ensure that we include a NULL at the end.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/options.c b/options.c
index 1ca4801..6462c67 100644
--- a/options.c
+++ b/options.c
@@ -550,7 +550,7 @@
 {
 	struct thread_data *td = data;
 	const char * const policy_types[] =
-		{ "default", "prefer", "bind", "interleave", "local" };
+		{ "default", "prefer", "bind", "interleave", "local", NULL };
 	int i;
 
 	char *nodelist = strchr(input, ':');