net: shell: dns: use int type for %.*s format specifier

The `%.*s` format specifier expects an int so cast the size_t parameter
to int.

Fixes an issue spotted in CI for:

    west twister -p native_sim/native/64 -s sample.net.dns_resolve.mdns

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>