netinet/udp.h: add TCP_ENCAP_ESPINTCP from linux v5.6

The use of TCP_ in udp.h is not known, fortunately udp.h is not
specified by posix so there are no strict namespace rules, added in

  linux commit e27cca96cd68fa2c6814c90f9a1cfd36bb68c593
  xfrm: add espintcp (RFC 8229)

include/netinet/udp.h

@@ -35,6 +35,7 @@ struct udphdr {
 #define UDP_ENCAP_GTP0		4
 #define UDP_ENCAP_GTP1U		5
 #define UDP_ENCAP_RXRPC		6
+#define TCP_ENCAP_ESPINTCP	7
 
 #define SOL_UDP            17