iptraf: Explicitly set EXTRA_OEMAKE as required

This recipe currently relies on EXTRA_OEMAKE having been to set to "-e
MAKEFLAGS=" in bitbake.conf to operate. It is necessary to make this
explicit so that the default in bitbake.conf can be changed.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>