checkpatch: allow spaces before parenthesis for 'coroutine_fn'

Signed-off-by: Jeff Cody <jcody@redhat.com>
Message-Id: <83b0fae0728906e18849c971d22d077d7fc0f179.1478010883.git.jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
