Add cstddef for ptrdiff_t

(cherry picked from commit 16dfc04da6d6fe92a3fbc3a2d0fa813861ada157)

Change-Id: I8aee60db9d563cbc93286f7e27d49c92035fe856
diff --git a/src/stringpiece.h b/src/stringpiece.h
index 7299120..7ff826d 100644
--- a/src/stringpiece.h
+++ b/src/stringpiece.h
@@ -30,6 +30,7 @@
 
 #include <string.h>
 #include <algorithm>
+#include <cstddef>
 #include <iosfwd>
 #include <string>