Move stringpiece.h and stringprintf.h to base/.

Change-Id: I7f71b4a12f99c5f81771146c66629ae5a947b229
diff --git a/src/atomic.cc b/src/atomic.cc
index 480e456..5bbbb4f 100644
--- a/src/atomic.cc
+++ b/src/atomic.cc
@@ -17,11 +17,12 @@
 #include "atomic.h"
 
 #include <pthread.h>
+
 #include <vector>
 
 #include "base/mutex.h"
 #include "base/stl_util.h"
-#include "stringprintf.h"
+#include "base/stringprintf.h"
 #include "thread.h"
 
 #if defined(__APPLE__)