binman: btool: gzip: fix packer name so that binary can be found

The binary is looked on the system by the suffix of the packer class.
This means binman was looking for btool_gzip on the system and not gzip.

Therefore, let's pass "gzip" as the name so that it can be found and
used.

Fixes: 0f369d79 ("binman: Add gzip bintool")
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>