gzip: default level with ENABLE_FEATURE_GZIP_LEVELS should be 6

Fixes an off-by-one that actually resulted in level 7 being used

Signed-off-by: Daniel Edgecumbe <git@esotericnonsense.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed