Fix compiler warning and cast const away.

TLVP_VAL returns a const u_int8_t* and we assign it
to a non const, cast const away.
diff --git a/openbsc/src/abis_rsl.c b/openbsc/src/abis_rsl.c
index 800f202..da12d93 100644
--- a/openbsc/src/abis_rsl.c
+++ b/openbsc/src/abis_rsl.c
@@ -971,7 +971,7 @@
 	}
 	if (TLVP_PRESENT(&tp, RSL_IE_L3_INFO)) {
 		DEBUGPC(DMEAS, "L3\n");
-		msg->l3h = TLVP_VAL(&tp, RSL_IE_L3_INFO);
+		msg->l3h = (u_int8_t *) TLVP_VAL(&tp, RSL_IE_L3_INFO);
 		return gsm0408_rcvmsg(msg, 0);
 	} else
 		DEBUGPC(DMEAS, "\n");