staging: greybus: loopback_test: Add blank line after declaration.

Declarations are to be followed by a blank line. Add to follow Linux
Kernel coding style. Issue reported by checkpatch.

Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Jack Schofield <schofija@oregonstate.edu>
Link: https://lore.kernel.org/r/048967b5c3b157e19ccf4608f71da8418fc31877.1669872193.git.schofija@oregonstate.edu


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>