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

Change-Id: I7f71b4a12f99c5f81771146c66629ae5a947b229
diff --git a/src/runtime_linux.cc b/src/runtime_linux.cc
index 862a066..0be6339 100644
--- a/src/runtime_linux.cc
+++ b/src/runtime_linux.cc
@@ -22,7 +22,7 @@
 
 #include "base/logging.h"
 #include "base/mutex.h"
-#include "stringprintf.h"
+#include "base/stringprintf.h"
 #include "thread.h"
 #include "utils.h"