heimdal: disable parallel build

It still seems to have issues, so disable it pending more patches and
testing. Fixes:
http://autobuild.buildroot.net/results/a07/a07198b3f7f8029d80863a998814877ec9994c60/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>