Declare clock_settime for macOS.

Author: DimitrisJim

libc-test/semver/macos.txt

@@ -1 +1,2 @@
+clock_settime
 memmem

src/unix/bsd/apple/mod.rs

@@ -5372,6 +5372,7 @@ pub unsafe fn mach_task_self() -> ::mach_port_t {
 cfg_if! {
     if #[cfg(target_os = "macos")] {
         extern "C" {
+            pub fn clock_settime(clock_id: ::clockid_t, tp: *const ::timespec) -> ::c_int;
             pub fn memmem(
                 haystack: *const ::c_void,
                 haystacklen: ::size_t,