ipa: Add E1INP_SIGN_OSMO to use IPAC_PROTO_OSMO via e1_input

In order to make use of the IPAC_PROTO_OSMO stream_id of the IPA
multiplex via the e1_input abstraction layer, we need to define
E1INP_SIGN_OSMO and treat it like other signalling (OML/RSL).

This is required for the upcoming code that uses IPAC_PROTO_OSMO
to remotely insert routing table entries into the OML router (e.g. from
osmo-bts).
diff --git a/src/e1_input.c b/src/e1_input.c
index e5ddaa3..0994910 100644
--- a/src/e1_input.c
+++ b/src/e1_input.c
@@ -205,6 +205,7 @@
 	[E1INP_SIGN_NONE]	= "None",
 	[E1INP_SIGN_OML]	= "OML",
 	[E1INP_SIGN_RSL]	= "RSL",
+	[E1INP_SIGN_OSMO]	= "OSMO",
 };
 const char *e1inp_signtype_name(enum e1inp_sign_type tp)
 {