Make stringpiece.h #include <algorithm> for std::min().

Fixes #33.

Change-Id: I9d283648c10fe84b96652de56c7d6274117d4dfc
Reviewed-on: https://code-review.googlesource.com/2940
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/re2/stringpiece.h b/re2/stringpiece.h
index 37c73d1..bc8bf40 100644
--- a/re2/stringpiece.h
+++ b/re2/stringpiece.h
@@ -20,6 +20,7 @@
 #define STRINGS_STRINGPIECE_H__
 
 #include <string.h>
+#include <algorithm>
 #include <cstddef>
 #include <iosfwd>
 #include <string>