modpost: require a MODULE_DESCRIPTION()

Since commit 1fffe7a3 ("script: modpost: emit a warning when the
description is missing"), a module without a MODULE_DESCRIPTION() has
resulted in a warning with make W=1. Since that time, all known
instances of this issue have been fixed. Therefore, now make it an
error if a MODULE_DESCRIPTION() is not present.

Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>