modpost: do not warn about missing MODULE_DESCRIPTION() for vmlinux.o

Building with W=1 incorrectly emits the following warning:

  WARNING: modpost: missing MODULE_DESCRIPTION() in vmlinux.o

This check should apply only to modules.

Fixes: 1fffe7a3

 ("script: modpost: emit a warning when the description is missing")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>