Add pthread_sigqueue.

pthread_sigqueue is a gnu libc extension, however I have a use case for it,
hence why I'm adding it.

src/unix/linux_like/mod.rs

@@ -1729,6 +1729,7 @@ extern "C" {
         val: *mut ::c_int,
     ) -> ::c_int;
     pub fn pthread_rwlockattr_setpshared(attr: *mut pthread_rwlockattr_t, val: ::c_int) -> ::c_int;
+    pub fn pthread_sigqueue(thread: ::pthread_t, sig: ::c_int, value: ::sigval) -> ::c_int;
     pub fn ptsname_r(fd: ::c_int, buf: *mut ::c_char, buflen: ::size_t) -> ::c_int;
     pub fn clearenv() -> ::c_int;
     pub fn waitid(idtype: idtype_t, id: id_t, infop: *mut ::siginfo_t, options: ::c_int)