ipaccess: Add new osmo extension IPAC_PROTO_EXT_PCU

This new extension protocol is used to forward Osmocom PCUIF messages
BSC<->BTS<->PCU.
It will be sent re-using the IPA multiplex of the OML link between
BSC and BTS. BTS is responsible for forwarding the message over the unix
socket to the PCU.

Related: SYS#5303
Change-Id: I68b04def49946b6915dbd4e476999f249751cd28
diff --git a/include/osmocom/gsm/protocol/ipaccess.h b/include/osmocom/gsm/protocol/ipaccess.h
index 4f1d0b1..5182760 100644
--- a/include/osmocom/gsm/protocol/ipaccess.h
+++ b/include/osmocom/gsm/protocol/ipaccess.h
@@ -39,6 +39,7 @@
 	IPAC_PROTO_EXT_GSUP	= 0x05,		/* GSUP GPRS extension */
 	IPAC_PROTO_EXT_OAP	= 0x06,		/* Osmocom Authn Protocol */
 	IPAC_PROTO_EXT_RSPRO	= 0x07,		/* Remote SIM protocol */
+	IPAC_PROTO_EXT_PCU	= 0x08,		/* BSC<->BTS<->PCU communication */
 };
 
 enum ipaccess_msgtype {