configure: fix libnuma_v2 probe to work with -Werror

I noticed that libnuma_v2 wasn't getting detected in our build
environment because we would get an error that mask was unused.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/configure b/configure
index 6e1376f..892335b 100755
--- a/configure
+++ b/configure
@@ -887,7 +887,7 @@
 int main(int argc, char **argv)
 {
   struct bitmask *mask = numa_parse_nodestring(NULL);
-  return 0;
+  return mask->size == 0;
 }
 EOF
 if compile_prog "" "" "libnuma api"; then