BSSGP: constify argument 'tp' of gprs_bssgp_pcu_rx_paging_{cs,ps}

Change-Id: I24e48964a0ff86c2ca962ab3928d07b1c9200390
diff --git a/src/gprs_bssgp_pcu.cpp b/src/gprs_bssgp_pcu.cpp
index 679f281..413408d 100644
--- a/src/gprs_bssgp_pcu.cpp
+++ b/src/gprs_bssgp_pcu.cpp
@@ -201,7 +201,7 @@
 	return 0;
 }
 
-static int gprs_bssgp_pcu_rx_paging_cs(struct msgb *msg, struct tlv_parsed *tp)
+static int gprs_bssgp_pcu_rx_paging_cs(struct msgb *msg, const struct tlv_parsed *tp)
 {
 	struct osmo_mobile_identity mi;
 	int rc;
@@ -212,7 +212,7 @@
 	return BTS::main_bts()->add_paging(tlvp_val8(tp, BSSGP_IE_CHAN_NEEDED, 0), &mi);
 }
 
-static int gprs_bssgp_pcu_rx_paging_ps(struct msgb *msg, struct tlv_parsed *tp)
+static int gprs_bssgp_pcu_rx_paging_ps(struct msgb *msg, const struct tlv_parsed *tp)
 {
 	struct osmo_mobile_identity mi_imsi;
 	struct osmo_mobile_identity paging_mi;