openbsd add getthrid fn.

Author: devnexen

libc-test/semver/openbsd.txt

@@ -968,6 +968,7 @@ getrusage
 getservbyport
 getservent
 getsid
+getthrid
 glob
 glob_t
 globfree

src/unix/bsd/netbsdlike/openbsd/mod.rs

@@ -1536,6 +1536,7 @@ extern "C" {
         timeout: *const ::timespec,
     ) -> ::c_int;
     pub fn mprotect(addr: *mut ::c_void, len: ::size_t, prot: ::c_int) -> ::c_int;
+    pub fn getthrid() -> ::pid_t;
     pub fn pthread_attr_getguardsize(
         attr: *const ::pthread_attr_t,
         guardsize: *mut ::size_t,