Add missing #include for PATH_MAX

Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>

--- memstat-0.8.org/memstat.c	2009-06-08 14:38:05.000000000 +0200
+++ memstat-0.8/memstat.c	2014-09-12 03:55:43.096035374 +0200
@@ -19,6 +19,7 @@
 #include <unistd.h>
 #include <getopt.h>
 #include <errno.h>
+#include <limits.h>
 
 /* blacklist devices that just map physical memory */
 char *blacklist[] = { "/dev/mem",
