Fix compilation for >=linux-headers-2.6.35

--- a/xhkeys_cdaudio.c
+++ b/xhkeys_cdaudio.c
@@ -16,6 +16,7 @@
 #include <string.h>
 #include <stdarg.h>
 #include <ctype.h>
+#include <limits.h>
 #include <linux/cdrom.h>
 #include <sys/ioctl.h>
 #include <fcntl.h>