SI: add missing header

Previous SI10 patch added function without exposing it via public header.
Let's fix this.

Fixes: 600d4eeab71665c0aec0a11a226d231f109534ca
Change-Id: Ia7530e9c8a21f6f99f3aac7baea5cbb38763c4f3
diff --git a/include/osmocom/gsm/gsm48.h b/include/osmocom/gsm/gsm48.h
index 2a93c5e..93a9e21 100644
--- a/include/osmocom/gsm/gsm48.h
+++ b/include/osmocom/gsm/gsm48.h
@@ -35,6 +35,7 @@
 const char *gsm48_cc_state_name(uint8_t state);
 const char *gsm48_cc_msg_name(uint8_t msgtype);
 const char *gsm48_rr_msg_name(uint8_t msgtype);
+const char *gsm48_rr_short_pd_msg_name(uint8_t msgtype);
 const char *rr_cause_name(uint8_t cause);
 const char *osmo_rai_name(const struct gprs_ra_id *rai);
 char *osmo_rai_name_buf(char *buf, size_t buf_len, const struct gprs_ra_id *rai);