Tidy up the string utilities.

Change-Id: Ifd19a737b4a6a3e333aac64909a4694f7169786e
Reviewed-on: https://code-review.googlesource.com/6832
Reviewed-by: Paul Wankadia <[email protected]>
diff --git a/re2/tostring.cc b/re2/tostring.cc
index b967198..8d2fd78 100644
--- a/re2/tostring.cc
+++ b/re2/tostring.cc
@@ -9,6 +9,7 @@
 #include <string>
 
 #include "util/util.h"
+#include "util/strutil.h"
 #include "re2/regexp.h"
 #include "re2/walker-inl.h"