Shorten enum names in mupdf_sys

[ginnyTheCat]

This prevents bindgen to prepend the enum name, as mupdf already does this themselves.

This turns fz_linejoin_FZ_LINEJOIN_ROUND for example to FZ_LINEJOIN_ROUND. bindgen also would have the option to turn it into fz_linejoin::FZ_LINEJOIN_ROUND (where fz_linejoin would be a module, but I think removing the prefix is the better choice.