package/nut: don't build in parallel

package/nut: don't build in parallel

The conditions are not trivial to reproduce, but it can happen that the
headers are not fully regenerated by the time they are included.

This only happens when a python and/or perl interpreter are available,
with a high number of jobs and a high load.

Fixes:
    http://autobuild.buildroot.org/results/523/5231ff39a0839ec5e1962662004214d4b0773068/
    http://autobuild.buildroot.org/results/5a7/5a75d44c028e77b58f0fd9ab794952f2b477dd84/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>