-pedantic does not like that (R_xlen_t vs `size_t)

`data` is a `std::vector` anyway, so I don't see the point of using `R_xlen_t`.

Author: romainfrancois

inst/include/Rcpp/Benchmark/Timer.h

@@ -110,7 +110,7 @@ namespace Rcpp{
         }
 
         operator SEXP() const {
-            R_xlen_t n = data.size();
+            size_t n = data.size();
             NumericVector out(n);
             CharacterVector names(n);
             for (size_t i=0; i<n; i++) {