libqcdm,ipv6pref: fix warnings with -Wdiscarded-qualifiers

ipv6pref.c: In function ‘main’:
  ipv6pref.c:288:11: warning: passing argument 1 of ‘putenv’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
    288 |   putenv ("QCDM_DEBUG=1");
        |           ^~~~~~~~~~~~~~

libqcdm/tests/ipv6pref.c

@@ -285,7 +285,7 @@ main (int argc, char *argv[])
 	}
 
 	if (debug)
-		putenv ("QCDM_DEBUG=1");
+		putenv ((char *)"QCDM_DEBUG=1");
 
 	fd = com_setup (dmport);
 	if (fd < 0)