package/benejson: only buid static/shared as needed

Fixes:
    http://autobuild.buildroot.org/results/56a/56a721885c890f3d62bf9e13d712767e66937c51/
    http://autobuild.buildroot.org/results/7c5/7c54dce4801a53bec378b422edfa42dfa990eabe/


    [...]

Patch-from: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>