-Wsign-conversion on iOS

We build Qt for iOS and got this warning if we build our own application. Since we have -Werror enabled this breaks our build.

In file included from /Users/.../lib/QtCore.framework/Headers/qplugin.h:45:
In file included from /Users/.../lib/QtCore.framework/Headers/qjsonobject.h:43:
/Users/.../lib/QtCore.framework/Headers/qjsonvalue.h:176:45: warning: implicit conversion changes signedness: 'int' to 'uint' (aka 'unsigned int') [-Wsign-conversion]
: a(array), is_object(false), index(idx) {}
_^~
/Users/.../lib/QtCore.framework/Headers/qjsonvalue.h:178:45: warning: implicit conversion changes signedness: 'int' to 'uint' (aka 'unsigned int') [-Wsign-conversion]
: o(object), is_object(true), index(idx) {}