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.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>