package/dash: don't build in paralle

package/dash: don't build in paralle

dash has races in its Makefile, but upstream is not too keen in fixing
those, and just suggests that dash not be built in parallel:

    https://www.mail-archive.com/dash@vger.kernel.org/msg01675.html

Fixes:
    http://autobuild.buildroot.org/results/793/7934c815a3009af688c2f1183e67dfe542c9a009/
    http://autobuild.buildroot.org/results/fc4/fc4e4ab47455ac47dd4a3a60083cec2848e74dbb/
    http://autobuild.buildroot.org/results/6bf/6bfdd44ea5d55a6ca3ef92254eab18c7c0416b7b/
    [...]

Reported-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>