cygwin: fix cpuset_t typo in CPU_ZERO

[phdye]

Description

CPU_ZERO in src/unix/cygwin/mod.rs:1712 declares its parameter as &mut cpuset_t, but no type by that name exists in the cygwin module. The struct is cpu_set_t (defined at line 91), and every sibling function (CPU_SET, CPU_CLR, CPU_ISSET, CPU_COUNT, CPU_COUNT_S, CPU_EQUAL) correctly uses cpu_set_t.

The typo prevents the cygwin module from compiling on x86_64-pc-cygwin:

error[E0425]: cannot find type `cpuset_t` in this scope
    --> src/unix/cygwin/mod.rs:1712:34
     |
1712 |     pub fn CPU_ZERO(cpuset: &mut cpuset_t) -> () {
     |                                  ^^^^^^^^

Sources

Internal — typo in this crate. The cpu_set_t struct is the only definition; no upstream header reference needed.

Checklist

• Relevant tests in libc-test/semver have been updated — N/A; not adding/removing symbols.
• No placeholder or unstable values like *LAST or *MAX are included
• Tested locally — with this fix applied on main, cargo test --target x86_64-pc-cygwin builds and passes the full suite (2968 ctest cases, all other suites green).

@rustbot label +stable-nominated

[phdye]

Companion PR for the same Cygwin port: #5099 (off-by-one in utsname.sysname length). The two are independent but libc-test on x86_64-pc-cygwin only runs end-to-end with both fixes applied — this PR is the prerequisite that lets the cygwin module compile, and #5099 then passes the suite.