ipa: Add function declaration for IPA header creation

These functions are called from OpenBSC and are supposed to be
public API of libosmo-abis.
diff --git a/include/osmocom/abis/ipa.h b/include/osmocom/abis/ipa.h
index 982b694..cce2c42 100644
--- a/include/osmocom/abis/ipa.h
+++ b/include/osmocom/abis/ipa.h
@@ -70,4 +70,8 @@
 
 int ipaccess_rcvmsg_base(struct msgb *msg, struct osmo_fd *bfd);
 
+void ipaccess_prepend_header(struct msgb *msg, int proto);
+void ipaccess_prepend_header_ext(struct msgb *msg, int proto);
+
+
 #endif