Add missing #include <stdlib.h>

After turning on exceptions in test tools,
iOS builds of ok started saying atoi() was not declared.

I don't even want to know.

Change-Id: I52f354a1f25ec042bf2161a4c5dd9276aa25e46a
Reviewed-on: https://skia-review.googlesource.com/34961
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

tools/ok_srcs.cpp

@@ -15,6 +15,7 @@
 #include <algorithm>
 #include <chrono>
 #include <limits>
+#include <stdlib.h>
 #include <vector>
 
 struct GMStream : Stream {