net: openthread: Use newlib in samples

OpenThread throws a warning when minimal libc is used, as some of its
new files use stdlib functions not available in the minimal
implementation. It's not critical failure, as those new functions are
not linked anyway in default configuration, but the warnings do not
look well. Therefore, use newlib by default in the sample to prevent
warnings.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>