Move gprs_sndcp_vty_init() declaration to gprs_sndcp.h

Change-Id: Iea9692e7ef9bd017d89ef654d2f2ae5b30cc4550
diff --git a/src/sgsn/sgsn_main.c b/src/sgsn/sgsn_main.c
index 36ff78d..362e7aa 100644
--- a/src/sgsn/sgsn_main.c
+++ b/src/sgsn/sgsn_main.c
@@ -59,6 +59,7 @@
 #include <osmocom/sgsn/vty.h>
 #include <osmocom/sgsn/sgsn.h>
 #include <osmocom/sgsn/gprs_llc.h>
+#include <osmocom/sgsn/gprs_sndcp.h>
 #include <osmocom/sgsn/gprs_gmm.h>
 #include <osmocom/sgsn/gprs_ranap.h>
 #include <osmocom/sgsn/gprs_ns.h>