nat: Rename "struct sccp_connections" to "struct nat_sccp_connection"

The name sccp_connection is used in the osmo-sccp code, sccp_connections
was used in the NAT for tracking a sccp_connection. Rename it so it is
obvious that the struct belongs to the nat.

The rename was done with sed:
$ sed -i s,"struct sccp_connections","struct nat_sccp_connection",g \
		include/openbsc/*.h src/osmo-bsc_nat/* tests/*/*
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c b/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
index e5993b4..46dc38e 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
@@ -178,7 +178,7 @@
       SHOW_STR "Display information about SCCP\n"
       "All active connections\n")
 {
-	struct sccp_connections *con;
+	struct nat_sccp_connection *con;
 	vty_out(vty, "Listing all open SCCP connections%s", VTY_NEWLINE);
 
 	llist_for_each_entry(con, &_nat->sccp_connections, list_entry) {