sys/resource.h: add rusage compatibility fields, set to zero

POSIX defines this structure but specifies only two fields (which we
already implement). However, both the *BSD and Linux have agreed on
some more fields, which are often assumed to be there by applications.

The benefice of having compatibility fields is
greater as having to patch every other software at HaikuPorts.

Change-Id: Ie28ca2e348aa16b4c57eb3498eb62175100d9b9d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4083
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Niels Sascha Reedijk <niels.reedijk@gmail.com>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>