fix from upstream to restore missing symbol

--- pcre_stringpiece.h.in	(revision 1499)
+++ pcre_stringpiece.h.in	(revision 1500)
@@ -174,6 +174,6 @@ template<> struct __type_traits<pcrecpp:
 #endif
 
 // allow StringPiece to be logged
-std::ostream& operator<<(std::ostream& o, const pcrecpp::StringPiece& piece);
+PCRECPP_EXP_DECL std::ostream& operator<<(std::ostream& o, const pcrecpp::StringPiece& piece);
 
 #endif /* _PCRE_STRINGPIECE_H */