fix(kvm): handle the pthread_t definition in musl

`pthread_t` is defined as `struct __pthread *` in musl[1] and
`unsigned long int` in glibc[2]. Use `as _` to let rust compiler do
the conversion.

[1] http://git.musl-libc.org/cgit/musl/tree/include/alltypes.h.in
[2] https://elixir.bootlin.com/glibc/latest/source/sysdeps/nptl/bits/pthreadtypes.h#L27

Signed-off-by: Changyuan Lyu <changyuanl@google.com>

alioth/src/hv/kvm/vm.rs

@@ -146,7 +146,7 @@ impl Vm for KvmVm {
     }
 
     fn stop_vcpu<T>(_id: u32, handle: &JoinHandle<T>) -> Result<(), Error> {
-        ffi!(unsafe { libc::pthread_kill(handle.as_pthread_t(), SIGRTMIN()) })?;
+        ffi!(unsafe { libc::pthread_kill(handle.as_pthread_t() as _, SIGRTMIN()) })?;
         Ok(())
     }