netinet/udp.h: add UDP_GRO from linux v5.0

sockopt to enable gro for udp.
added in linux commit e20cf8d3f1f763ad28a9cb3b41305b8a8a42653e

include/netinet/udp.h

@@ -27,6 +27,7 @@ struct udphdr {
 #define UDP_NO_CHECK6_TX 101
 #define UDP_NO_CHECK6_RX 102
 #define UDP_SEGMENT	103
+#define UDP_GRO		104
 
 #define UDP_ENCAP_ESPINUDP_NON_IKE 1
 #define UDP_ENCAP_ESPINUDP	2