css.rs: Fix usize cast

Previously we were casting usize, returned from the .len() method,
to a u64 and later convert that to libc::c_ulong in the C bindings.

But libc::ulong is u64 in amd64, but represented as u32 in i686,
which results into a type missmatch whe compiled in i686.

Givem that libc::c_ulong seems to behave like usize, it should be
safe cast usize to libc::c_ulong directly.
3 jobs for <span class="ref-name">alatiera/ulong</span> in 6 minutes and 46 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #113032
fedora test

00:06:08

 
  Lint
manual #113034
allowed to fail manual
clippy
passed #113033
rustfmt

00:00:38