block_int: make bdrv_backing_overridden static

bdrv_backing_overridden is only used in block.c, so there is
no need to leave it in block_int.h

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20211215121140.456939-2-eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>