python-netifaces: use single version externally managed

We want --single-version-externally-managed (and --root=/) to make
python packages uncompressed (python eggs are zip files) since
compression should be a target filesystem responsibility.
And we don't need/want multiple versions of the same package in the
target since packages should be handled by buildroot on any sane
embedded system.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>