examples/ethtool: build as part of meson build

examples/ethtool: build as part of meson build

Since the code for the ethtool example was contained in subdirectories the
compilation of this example is different from most of the other apps, and
it had not been abled when the user requests a build with "-Dexamples=all".

To simplify the build with meson, the separate ethtool library is not build
separately, rather the app is built as a single entity.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Luca Boccassi <bluca@debian.org>