Include <sys/param.h> to get PATH_MAX definition

* pathtrace.c: Include <sys/param.h> instead of <limits.h> to get
PATH_MAX definition.

Reported-by: Steve Bennett <steveb@workware.net.au>
diff --git a/pathtrace.c b/pathtrace.c
index b2efd33..3ebb427 100644
--- a/pathtrace.c
+++ b/pathtrace.c
@@ -29,7 +29,7 @@
 #include "defs.h"
 
 #include <ctype.h>
-#include <limits.h>
+#include <sys/param.h>
 
 #ifdef HAVE_POLL_H
 #include <poll.h>