add missing call to libdhcp_callbacks_register().  should stop
dhcrelay() crashing when accessing *libdhcp_callbacks.local_port.

Index: dist/relay/dhcrelay.c
===================================================================
RCS file: /cvsroot/src/external/mpl/dhcp/dist/relay/dhcrelay.c,v
retrieving revision 1.2
diff -p -u -r1.2 dhcrelay.c
--- dist/relay/dhcrelay.c	7 Apr 2018 22:37:30 -0000	1.2
+++ dist/relay/dhcrelay.c	21 May 2021 01:58:14 -0000
@@ -310,6 +310,8 @@ main(int argc, char **argv) {
 	int local_family_set = 0;
 #endif
 
+	libdhcp_callbacks_register(&dhcrelay_callbacks);
+
 #ifdef OLD_LOG_NAME
 	progname = "dhcrelay";
 #else