Fix an MSVC warning in onion.h prototypes

src/or/onion.h

@@ -101,9 +101,9 @@ typedef struct extended_cell_t {
 
 int create_cell_parse(create_cell_t *cell_out, const cell_t *cell_in);
 int created_cell_parse(created_cell_t *cell_out, const cell_t *cell_in);
-int extend_cell_parse(extend_cell_t *cell_out, uint8_t command,
+int extend_cell_parse(extend_cell_t *cell_out, const uint8_t command,
                       const uint8_t *payload_in, size_t payload_len);
-int extended_cell_parse(extended_cell_t *cell_out, uint8_t command,
+int extended_cell_parse(extended_cell_t *cell_out, const uint8_t command,
                         const uint8_t *payload_in, size_t payload_len);
 
 int create_cell_format(cell_t *cell_out, const create_cell_t *cell_in);