apparmor: remove unneeded #ifdef in decompress_zstd()

The whole function is guarded by CONFIG_SECURITY_APPARMOR_EXPORT_BINARY,
so the #ifdef here is redundant, remove it.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>