Revert "heimdal: disable parallel make"

This reverts commit f6980126.

Parallel make was disabled back in 2017:
https://git.buildroot.net/buildroot/commit/?id=f6980126f26c63421d04a32b6710d9e3a380476a



Build-test with -j99 was successful.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>