scx_utils: Remove unused assignment warning

[EricccTaiwan]

Initialize raw_capacity only when needed to avoid warning.

Ref: #1893

Log:

warning: value assigned to `raw_capacity` is never read
   --> rust/scx_utils/src/topology.rs:606:13
    |
606 |     let mut raw_capacity = 0;
    |             ^^^^^^^^^^^^
    |
    = help: maybe it is overwritten before being read?
    = note: `#[warn(unused_assignments)]` on by default

warning: `scx_utils` (lib) generated 1 warning

[multics69]

Good catch!