Add ppoll() for all unix platforms.

I'm unsure of whether there is support in OS X for this, and I can't find
anything online (I'm betting there isn't), but I'm going to let this run
through CI to confirm

Author: Susurrus

src/unix/mod.rs

@@ -721,6 +721,9 @@ extern {
     #[cfg_attr(all(target_os = "macos", target_arch = "x86"),
                link_name = "poll$UNIX2003")]
     pub fn poll(fds: *mut pollfd, nfds: nfds_t, timeout: ::c_int) -> ::c_int;
+    #[cfg_attr(all(target_os = "macos", target_arch = "x86"),
+               link_name = "ppoll$UNIX2003")]
+    pub fn ppoll(fds: *mut pollfd, nfds: nfds_t, timeout: ::c_int, sigmask: *const sigset_t) -> ::c_int;
     #[cfg_attr(all(target_os = "macos", target_arch = "x86_64"),
                link_name = "select$1050")]
     #[cfg_attr(all(target_os = "macos", target_arch = "x86"),