Add gettid() for Linux

Fixes #2076

Author: TheDoctor314

src/unix/linux_like/mod.rs

@@ -1666,6 +1666,7 @@ extern "C" {
         flags: ::c_int,
     ) -> ::ssize_t;
     pub fn uname(buf: *mut ::utsname) -> ::c_int;
+    pub fn gettid() -> ::pid_t;
 }
 
 cfg_if! {