[LyX/master] Add missing header

[Guillaume Munch]

commit 8e4648f38c2b6051832faf2e31e280ba36e3184a
Author: Guillaume Munch <g...@lyx.org>
Date:   Sat Jun 11 05:08:23 2016 +0100

    Add missing header
    
    Spotted by jkulesza

diff --git a/src/support/filetools.cpp b/src/support/filetools.cpp
index a72c93e..054a5ff 100644
--- a/src/support/filetools.cpp
+++ b/src/support/filetools.cpp
@@ -42,6 +42,7 @@
 #include "support/lassert.h"
 #include "support/regex.h"
 
+#include <limits.h>
 #include <fcntl.h>
 #ifdef HAVE_MAGIC_H
 #include <magic.h>