[trac613] supporess 'debug' output from cppcheck

with it cppcheck on bind10.isc.org would complain about lots of include file
'not found'.

src/cppcheck-suppress.lst

@@ -2,3 +2,4 @@ unreadVariable:src/lib/dns/rdata/template.cc:59
 uninitVar:src/lib/nsas/nameserver_entry.h
 functionConst:src/lib/cache/message_cache.h
 functionConst:src/lib/cache/rrset_cache.h
+debug