misc: Deal with smatch warnings about the bitfields

Use unsigned ints for the bitfield.
diff --git a/openbsc/include/openbsc/gprs_ns.h b/openbsc/include/openbsc/gprs_ns.h
index f0ec26e..7045d84 100644
--- a/openbsc/include/openbsc/gprs_ns.h
+++ b/openbsc/include/openbsc/gprs_ns.h
@@ -154,7 +154,7 @@
 	struct {
 		struct osmo_fd fd;
 		uint32_t local_ip;
-		int enabled:1;
+		unsigned int enabled:1;
 	} frgre;
 };