package/jsoncpp: Do not treat conversion warning as error

package/jsoncpp: Do not treat conversion warning as error

Fixes
http://autobuild.buildroot.net/results/74b/74b1b9f8defc275ff2bc3d877faf2863929737b9/
http://autobuild.buildroot.net/results/71d/71dcaf18e3d42c66af8036a68250eea1be09227b/
http://autobuild.buildroot.net/results/599/5997a9e4a8ca8b6171438495920d0a487518851c/
http://autobuild.buildroot.net/results/4ab/4abde1d7fd631d3b50a6b53824a781a08d022bca/
http://autobuild.buildroot.net/results/968/9680ebd8c804ef15d471519d54f8aa209710414d/
http://autobuild.buildroot.net/results/d5b/d5be4d51458ca10f3f4ff5fd3ad4b3cfd097f3ad/
http://autobuild.buildroot.net/results/b1e/b1e3fc27f3d0cdd6ca8f7aff06f7d5e4108817ec/
http://autobuild.buildroot.net/results/ffb/ffb40572269c63438228000495e1dc78c778788e/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>