patman: Handle non-ascii characters in names

When gathering addresses for the Cc list patman would encounter a
UnicodeDecodeError due to non-ascii characters in the author name.
Address this by explicitly using utf-8 when building the Cc list.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>