lib/oe/gpg_sign.py: Fix gpg verification

A stray space made it into the command for verifying gpg signatures.
This caused verification to fail, at least on my host. Removing the
space makes it work as expected.

Signed-off-by: Daniel McGregor <daniel.mcgregor@vecima.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>