Merge pull request rust-lang#3690 from tesuji/android-posix-spawn

Re-add `posix_spawn{_file_actions_t,attr_t}` on Android

Author: the8472

libc-test/build.rs

@@ -1681,6 +1681,7 @@ fn test_android(target: &str) {
                "sched.h",
                "semaphore.h",
                "signal.h",
+               "spawn.h",
                "stddef.h",
                "stdint.h",
                "stdio.h",