Declare std::string in C++ code, since re2.h has stopped doing so.

src/vre2/vre2.h

@@ -36,6 +36,7 @@
 #include <re2/re2.h>
 
 using namespace re2;
+using std::string;
 
 class vre2 {
 private:

src/vre2/vre2set.h

@@ -44,6 +44,7 @@ typedef enum {
 #include <re2/set.h>
 
 using namespace re2;
+using std::string;
 
 class vre2set {
 private: