patman: fix coverletter cc list with nulls

fixes: 8ab452d5

When compiling list of cover letter cc addresses, using null as a
separater, then encoding to utf-8 results in lots of "\x00" as
separators. patman then doesnt understand that when it comes to
repoting the list to send-email.

Fix this by not encoding to utf-8, as done for the other patch files.

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>