package/json-for-modern-cpp: relax dependency on cmake version

package/json-for-modern-cpp: relax dependency on cmake version

Currently, json-for-modern-cpp requires cmake to be at least cmake-3.8,
but our dependencies only check for cmake-3.1.

It turns out that json-for-modern-cpp is over strict, and could very
well do with just cmake-3.1

Fixes:
    http://autobuild.buildroot.org/results/1f8/1f893d8bc7df3bfefd31df69aba918e2121cfaf7/

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>