cmd: license: Add CONFIG_GZIP dependency

'License' command processing code could be successfully compiled only
when CONFIG_GZIP option is enabled, otherwise it can't find the 'gunzip'
function definition (it is defined in lib/gunzip.c).

Add CONFIG_GZIP dependency to 'license' command config option in the
corresponding Kconfig.

Signed-off-by: Ivan Orlov <ivan.orlov@codethink.co.uk>