Add the missing definition for StringPiece::AppendToString().

Fixes #49.

Change-Id: Icbacbad6b7dae4681f0367f381ac910f4cf6817b
Reviewed-on: https://code-review.googlesource.com/3421
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/re2/stringpiece.cc b/re2/stringpiece.cc
index f9e2294..c243527 100644
--- a/re2/stringpiece.cc
+++ b/re2/stringpiece.cc
@@ -33,6 +33,10 @@
   target->assign(ptr_, length_);
 }
 
+void StringPiece::AppendToString(string* target) const {
+  target->append(ptr_, length_);
+}
+
 int StringPiece::copy(char* buf, size_type n, size_type pos) const {
   int ret = min(length_ - pos, n);
   memcpy(buf, ptr_ + pos, ret);