Add cast to rta_getattr_str

Warning from C++
diff --git a/include/libnetlink.h b/include/libnetlink.h
index b0656ce..b85c377 100644
--- a/include/libnetlink.h
+++ b/include/libnetlink.h
@@ -87,7 +87,7 @@
 }
 static inline const char *rta_getattr_str(const struct rtattr *rta)
 {
-	return RTA_DATA(rta);
+	return (const char *)RTA_DATA(rta);
 }
 
 extern int rtnl_listen(struct rtnl_handle *, rtnl_filter_t handler,