ignore maybe-initialized warning in map_caster::convert_elements field conv

[alan-rager-skydio]

In map_caster::convert_elements, conv is initialized with a call to ::load, and GCC 13 sees that as a maybe-uninitialized error to call it later:

for (const auto &it : seq) {
    value_conv conv;              // line 332: declared, "uninitialized"
    if (!conv.load(it, convert)) { // line 333: conv.load() initializes conv's internal state
        return false;
    }
    value.push_back(cast_op<Value &&>(std::move(conv)));  // line 336: uses conv
}

By adding ignores around this area, we can avoid the compilation failure.