ob-C: use size_t in babel D

* lisp/ob-C.el (org-babel-C-header-to-C): change ulong to size_t for
  indexing a table as mandated by the latest version of the D language
  (thanks to Chris Andrews)

lisp/ob-C.el

@@ -431,7 +431,7 @@ specifying a variable with the name of the table."
 	 table table (length headers) table))
        (d
 	(format
-	 "string %s_h (ulong row, string col) { return %s[row][get_column_num(%s_header,col)]; }"
+	 "string %s_h (size_t row, string col) { return %s[row][get_column_num(%s_header,col)]; }"
 	 table table table))))))
 
 (provide 'ob-C)