Add a missing include in symlink_posix.cc

Change-Id: Ib51bcee8893c266cd9362c576b2aabd9bf563afe
diff --git a/filesystem/filesystem/symlink_posix.cc b/filesystem/filesystem/symlink_posix.cc
index 4b942ea..fddf036 100644
--- a/filesystem/filesystem/symlink_posix.cc
+++ b/filesystem/filesystem/symlink_posix.cc
@@ -5,6 +5,7 @@
 #include "filesystem/symlink.h"
 
 #include <limits.h>
+#include <stdlib.h>
 #include <unistd.h>
 
 #include "filesystem/build_config.h"