darwin: Replaced low-level mach API with pthread API

Found this because clang was warning that the return value was being
converted from unsigned to signed.

The need for a mach API surprised me, and I found this alternative
formulation in a Chromium bug:

https://groups.google.com/forum/#!topic/native-client-reviews/z---hlHbQNc

which describes: "mach_thread_self() needs to be balanced by mach_port_deallocate(),
which is two system calls. pthread_mach_thread_np(pthread_self()) is two libc
function calls and no system calls, because pthread caches the Mach port."

Fixed the conversion warning too, with a cast.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>

libusb/os/threads_posix.c

@@ -29,7 +29,7 @@
 # include <unistd.h>
 # include <sys/syscall.h>
 #elif defined(__APPLE__)
-# include <mach/mach.h>
+# include <pthread.h>
 #elif defined(__CYGWIN__)
 # include <windows.h>
 #endif
@@ -69,8 +69,7 @@ int usbi_get_tid(void)
 	   real thread support. For 5.1 and earlier, -1 is returned. */
 	ret = syscall(SYS_getthrid);
 #elif defined(__APPLE__)
-	ret = mach_thread_self();
-	mach_port_deallocate(mach_task_self(), ret);
+	ret = (int)pthread_mach_thread_np(pthread_self());
 #elif defined(__CYGWIN__)
 	ret = GetCurrentThreadId();
 #else

libusb/version_nano.h

@@ -1 +1 @@
-#define LIBUSB_NANO 11280
+#define LIBUSB_NANO 11281