apparmor: fix implicit declaration of function 'decompress_zstd'

When CONFIG_ZSTD_DECOMPRESS is not enabled, and neither
CONFIG_SECURITY_APPARMOR_EXPORT_BINARY nor
CONFIG_SECURITY_APPARMOR_COMPRESSED_POLICY are enabled.

The build will fail with implicit declaration of function
'decompress_zstd' because there is not an appropriate stub function,
for when the zstd decompression isn't enabled.

In addition fix compress_min, and compress_max to be conditional on
CONFIG_SECURITY_APPARMOR_EXPORT_BINARY, as they are used with the
exported policy.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202608010834.9yIVzhG2-lkp@intel.com/
Fixes: 1c5f27e845e84 ("apparmor: Fix build failure when ZSTD_DECOMPRESS is not enabled")
Signed-off-by: John Johansen <john.johansen@canonical.com>