apple: include proper mach primatives

Fixes implicit declaration warning for 'mach_task_self'. This change
is an update to Change I9991dedd1ccfddc092eca86705ecbc3b764b799d,
which fixed this issue for the decoder but not the encoder.

Change-Id: I9df033e81f9520c4f975b7a7cf6c643d12e87c96

vp8/common/threading.h

@@ -38,6 +38,7 @@
 #define pthread_self() GetCurrentThreadId()
 #else
 #ifdef __APPLE__
+#include <mach/mach_init.h>
 #include <mach/semaphore.h>
 #include <mach/task.h>
 #include <time.h>

vp8/decoder/threading.c

@@ -12,9 +12,6 @@
 #if !defined(WIN32) && CONFIG_OS_SUPPORT == 1
 # include <unistd.h>
 #endif
-#ifdef __APPLE__
-#include <mach/mach_init.h>
-#endif
 #include "onyxd_int.h"
 #include "vpx_mem/vpx_mem.h"
 #include "vp8/common/threading.h"