sgsn_libgtp: refactor ps paging into gprs_gb

Paging should be done in gprs_gb
Change-Id: I3020020f52005aa460ca6b245ee2297aec9821b3
diff --git a/include/osmocom/sgsn/gprs_gb.h b/include/osmocom/sgsn/gprs_gb.h
index 47f2a31..461eee3 100644
--- a/include/osmocom/sgsn/gprs_gb.h
+++ b/include/osmocom/sgsn/gprs_gb.h
@@ -9,3 +9,6 @@
 			   bool drop_cipherable);
 /* Has to be called whenever any PDU (signaling, data, ...) has been received */
 void gprs_gb_recv_pdu(struct sgsn_mm_ctx *mmctx);
+
+/* page a MS in its routing area */
+int gprs_gb_page_ps_ra(struct sgsn_mm_ctx *mmctx);