KEYS: encrypted: add missing MODULE_DESCRIPTION()

During kbuild, with W=1, modpost will warn when a module doesn't have
a MODULE_DESCRIPTION(). The encrypted-keys module does not have a
MODULE_DESCRIPTION().  But currently, even with an allmodconfig
configuration, this module is built-in, and as a result, kbuild does
not currently warn about the missing MODULE_DESCRIPTION().

However, just in case it is built as a module in the future, add the
missing MODULE_DESCRIPTION() macro invocation.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>