json-c: fix build without threads support

An uClibc toolchain that does not support threads emits a warning when
_REENTRANT is defined. This breaks build with -Werror. Remove -Werror as
a workaround to fix the build.

This issue as been reported upstream
(https://github.com/json-c/json-c/issues/396).

Fixes:
http://autobuild.buildroot.net/results/606/60683141f2ef320a28820c4cd65d2b951123b147/
http://autobuild.buildroot.net/results/969/969faba96d94c1bda7e06b005d8f3a3f7175a0a2/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>