Increase HS_CELL_INTRODUCE1_MIN_SIZE to 490

As per torspec!167 (merged), we should increase the padded intro cell size to 490, to avoid leaking which extensions are in use by a service (such as pow).

This is done by increasing the #define HS_CELL_INTRODUCE1_MIN_SIZE in hs_cell.h.

This seems safe to backport, but I'll put it in 0.4.9 to test in a sim there first.