Add missing include to test_support.h.

TBR=vardhan@google.com

Review URL: https://codereview.chromium.org/2252733002 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 980e67bb28941788d0aa2d312e33f519cfcaa09a
diff --git a/cpp/test_support/test_support.h b/cpp/test_support/test_support.h
index f571ab1..c4050d4 100644
--- a/cpp/test_support/test_support.h
+++ b/cpp/test_support/test_support.h
@@ -8,6 +8,8 @@
 #ifndef MOJO_PUBLIC_CPP_TEST_SUPPORT_TEST_SUPPORT_H_
 #define MOJO_PUBLIC_CPP_TEST_SUPPORT_TEST_SUPPORT_H_
 
+#include <stdio.h>
+
 #include <string>
 #include <vector>