qcow2: Remove forward declaration of QCowAIOCB

This struct doesn't exist any more since commit 3fc48d09 in August 2011,
it's about time to remove its forward declaration.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

block/qcow2.h

diff --git a/block/qcow2.h b/block/qcow2.h
index d700bf1..3512263 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -295,8 +295,6 @@
     char *image_backing_format;
 } BDRVQcow2State;
 
-struct QCowAIOCB;
-
 typedef struct Qcow2COWRegion {
     /**
      * Offset of the COW region in bytes from the start of the first cluster