fix -Werror error

Author: Qiang Kou

inst/include/Rcpp/String.h

@@ -407,9 +407,6 @@ namespace Rcpp {
         /** the CHARSXP this String encapsulates */
         SEXP data ;
 
-        /** the encoding of encapsulated CHARSXP */
-        cetype_t enc;
-
         /** a buffer used to do string operations withough going back to the SEXP */
         std::string buffer ;
 
@@ -419,6 +416,9 @@ namespace Rcpp {
         /** is the buffer initialized */
         bool buffer_ready ;
 
+        /** the encoding of encapsulated CHARSXP */
+        cetype_t enc;
+
         inline bool is_na() const { return data == NA_STRING ; }
         inline void setBuffer(){
             if( !buffer_ready){