benejson: fix for non-wchar toolchains

wchar needs to be disabled in order for it to build on !wchar. Fixes:
http://autobuild.buildroot.net/results/71f/71f19d5056b56d40463b9fef7fa859e0025c4991/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>