bind: fix intermittent build issues with high BR2_JLEVEL

bind: fix intermittent build issues with high BR2_JLEVEL

Build sometimes breaks with:

libtool: link: `unix/os.lo' is not a valid libtool object
make[3]: *** [rndc-confgen] Error 1
make[3]: *** Waiting for unfinished jobs....
make[4]: Leaving directory `/scratch/peko/build/bind-9.6-ESV-R4/bin/rndc/unix'

So disable parallel builds.

This patch was removed with commit
c36b5d89c5616f7ca0a7295cbb5c231606beb71e by Gustavo Zacarias
<gustavo@zacarias.com.ar> but the problem still occurs, so disabling
parallel builds again.

Fixes:
http://autobuild.buildroot.org/results/220/2201f04170ea8ef0961e907efce07c041a57c229/

Signed-off-by: Jan Heylen <heyleke@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>