move utils.h functions to libosmocore

This needs the corresponding commit in libosmocore which imports
the related functions
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat.c b/openbsc/src/osmo-bsc_nat/bsc_nat.c
index efae54f..f20b248 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat.c
@@ -48,7 +48,6 @@
 #include <openbsc/abis_nm.h>
 #include <openbsc/socket.h>
 #include <openbsc/vty.h>
-#include <openbsc/utils.h>
 
 #include <osmocom/ctrl/control_cmd.h>
 #include <osmocom/ctrl/control_if.h>
@@ -1023,7 +1022,7 @@
 		return 0;
 	}
 
-	return constant_time_cmp(vec.res, key, 8) == 0;
+	return osmo_constant_time_cmp(vec.res, key, 8) == 0;
 }
 
 static void ipaccess_auth_bsc(struct tlv_parsed *tvp, struct bsc_connection *bsc)