test/vsock: remove vsock_perf executable on `make clean`

We forgot to add vsock_perf to the rm command in the `clean`
target, so now we have a left over after `make clean` in
tools/testing/vsock.

Fixes: 8abbffd2

 ("test/vsock: vsock_perf utility")
Cc: AVKrasnov@sberdevices.ru
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org> # build-tested
Link: https://lore.kernel.org/r/20230803085454.30897-1-sgarzare@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>