YJIT: Fix a warning on cargo test

[k0kubun]

This PR fixes the following warning:

$ cargo test --all-features
   Compiling yjit v0.1.0 (/home/k0kubun/src/github.com/ruby/ruby/yjit)
warning: unused variable: `exit_pc`
   --> src/stats.rs:615:45
    |
615 | pub extern "C" fn rb_yjit_record_exit_stack(exit_pc: *const VALUE)
    |                                             ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_exit_pc`
    |
    = note: `#[warn(unused_variables)]` on by default

It's conditionally used for #[cfg(not(test))], so it currently becomes a warning on cargo test.