Move func defintions of funcs implemented in gprs_sndcp.c to gprs_sndcp.h

Move them to the correct header where they belong, so that the all the
related protocol stuff is placed together.

Change-Id: I9052f48a0af125bb445194f4ae94ebbe49508fda
diff --git a/src/sgsn/gprs_sgsn.c b/src/sgsn/gprs_sgsn.c
index cdfa029..9ce632c 100644
--- a/src/sgsn/gprs_sgsn.c
+++ b/src/sgsn/gprs_sgsn.c
@@ -49,6 +49,7 @@
 #include <osmocom/sgsn/gprs_mm_state_iu_fsm.h>
 #include <osmocom/sgsn/gprs_gmm_fsm.h>
 #include <osmocom/sgsn/gprs_llc.h>
+#include <osmocom/sgsn/gprs_sndcp.h>
 #include <osmocom/sgsn/gtp_ggsn.h>
 
 #include <pdp.h>