Fix build breaks on M

Fix "Undeclared identifier 'PATH_MAX'" on darwin.

Change-Id: Icb2085f66df3601918dca9b30f8de801e0b16acd
diff --git a/qcom/dtbtool/dtbtool.c b/qcom/dtbtool/dtbtool.c
index 9df62a9..94b85dc 100644
--- a/qcom/dtbtool/dtbtool.c
+++ b/qcom/dtbtool/dtbtool.c
@@ -39,6 +39,7 @@
 #include <getopt.h>
 #include <errno.h>
 #include <unistd.h>
+#include <limits.h>
 
 #define QCDT_MAGIC     "QCDT"  /* Master DTB magic */
 #define QCDT_VERSION   3       /* QCDT version */