Move sgsn_pdp_ctx to its own file pdpctx.{c,h}

This further shrinks the mess in gprs_sgsn.h, and allows to easily see
layer violations (like pdpctx.c requiring llc.h)

Change-Id: Iad4da06efee7d8514ff48423bdaebc0f26413cc1
diff --git a/src/sgsn/sgsn_vty.c b/src/sgsn/sgsn_vty.c
index 2313a9a..8e94041 100644
--- a/src/sgsn/sgsn_vty.c
+++ b/src/sgsn/sgsn_vty.c
@@ -42,6 +42,7 @@
 #include <osmocom/sgsn/gtp_ggsn.h>
 #include <osmocom/sgsn/gtp_mme.h>
 #include <osmocom/sgsn/vty.h>
+#include <osmocom/sgsn/pdpctx.h>
 #include <osmocom/gsupclient/gsup_client.h>
 
 #include <osmocom/vty/tdef_vty.h>