Fix implicit function declaration on Android

there's no fio_set_sched_idle in os/os-android.h. copy the one from
os/os-linux.h.

current Android also has posix_madvise.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/os/os-android.h b/os/os-android.h
index 6b074cd..172ad9a 100644
--- a/os/os-android.h
+++ b/os/os-android.h
@@ -38,6 +38,13 @@
 
 #define OS_MAP_ANON		MAP_ANONYMOUS
 
+#ifndef POSIX_MADV_DONTNEED
+#define posix_madvise   madvise
+#define POSIX_MADV_DONTNEED MADV_DONTNEED
+#define POSIX_MADV_SEQUENTIAL	MADV_SEQUENTIAL
+#define POSIX_MADV_RANDOM	MADV_RANDOM
+#endif
+
 #ifdef MADV_REMOVE
 #define FIO_MADV_FREE	MADV_REMOVE
 #endif