tools/elfutils: enable parallel building

tools/elfutils: enable parallel building

There are multiple subdirectories for elfutils
and more than one is not dependent on another,
so enabling parallel building can save time
if multiple cores are available.

The change in build time for the package
is as much as 20% when cores are not in use by other jobs.

Signed-off-by: Michael Pratt <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/16522
Signed-off-by: Robert Marko <[email protected]>