sms: Put the try_deliver into the header file

Even if it is using BSC/NITB types let's put it in the header
file than just declaring it at a place that could bitrot in a
way that doesn't lead a warning.
diff --git a/openbsc/src/libmsc/smpp_smsc.h b/openbsc/src/libmsc/smpp_smsc.h
index 21d28dd..8d31b2b 100644
--- a/openbsc/src/libmsc/smpp_smsc.h
+++ b/openbsc/src/libmsc/smpp_smsc.h
@@ -131,4 +131,12 @@
 int smpp_vty_init(void);
 
 int smpp_determine_scheme(uint8_t dcs, uint8_t *data_coding, int *mode);
+
+
+
+struct gsm_sms;
+struct gsm_subscriber_connection;
+
+int smpp_try_deliver(struct gsm_sms *sms,
+			    struct gsm_subscriber_connection *conn);
 #endif