nat_traversal: declare flag literal as unsigned

- GH #2954

(cherry picked from commit 8c30109df0b479cc3fce39a765717149e9c70ddf)
(cherry picked from commit 4814bac2424db7fea2ba817fcdf4594a7b2e1c11)

src/modules/nat_traversal/nat_traversal.c

@@ -85,7 +85,7 @@ MODULE_VERSION
 
 
 /* WARNING: Keep this aligned with parser/msg_parser.h! */
-#define FL_DO_KEEPALIVE (1 << 31)
+#define FL_DO_KEEPALIVE (1u << 31)
 
 #define HASH_SIZE 512