Add missing header for free function

Change-Id: I8ff2dcb546849493f2883777895341e043e44fb3
Reviewed-on: https://chromium-review.googlesource.com/981965
Reviewed-by: Mark Mentovai <mark@chromium.org>
diff --git a/src/common/path_helper.cc b/src/common/path_helper.cc
index 60fe34f..61a6e31 100644
--- a/src/common/path_helper.cc
+++ b/src/common/path_helper.cc
@@ -31,6 +31,7 @@
 
 #include <assert.h>
 #include <libgen.h>
+#include <stdlib.h>
 #include <string.h>
 
 namespace google_breakpad {