Remove unnecessary inclusions of <numbers.h>

Functions display_msg() and check_sync() are only used in numbers.c, so they
can be static.

Remove inclusions of <numbers.h> from .c files that don't need it.

source/irc.c

@@ -43,7 +43,6 @@ CVS_REVISION(irc_c)
 #include "history.h"
 #include "exec.h"
 #include "notify.h"
-#include "numbers.h"
 #include "mail.h"
 #include "debug.h"
 #include "newio.h"