thread-pool: Drop unnecessary includes

Dragging block_int.h into a header is *not* nice.  Fortunately, this
is the only offender.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

thread-pool.c

@@ -20,7 +20,6 @@
 #include "qemu/osdep.h"
 #include "block/coroutine.h"
 #include "trace.h"
-#include "block/block_int.h"
 #include "block/thread-pool.h"
 #include "qemu/main-loop.h"