fix building with newer glibc

http://bugs.gentoo.org/246734

--- a/cancd.c
+++ b/cancd.c
@@ -36,6 +36,7 @@
 #include <libgen.h>
 #include <getopt.h>
 #include <stdarg.h>
+#include <limits.h>
 #include <sys/socket.h>
 #include <arpa/inet.h>
 #include <netinet/in.h>