heimdal: disable parallel make

It still seems to suffer from issues. Fixes:
http://autobuild.buildroot.net/results/e21/e2176cb5adacdad14371d4828f13a1c23f0e18d5/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>