build: Fix dependency file inclusion

build: Fix dependency file inclusion

The objs.mk include changes for archive files broke things
completely and none of the dependency files (*.d) ever got
included, as the expansion there ended up empty.

Clearly, my mistake, I should've tested that better. As we
don't need the %.a files in the list there use filter-out
to remove them, rather than what I had lazily wanted to do,
which was trying to read %.d files for them. The filter-out
actually works, and avoids looking up files that can never
exist in the first place.

Fixes: 87098d3324e0 ("build: Put archive files into build/ folder too")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>