[libknet] bump soname due to ABI changes introduced in e1536067d2ade7c184e80b9fcc233d309f4d7c01

soname 2.0.0 will stay around till stable2 is branched.
followers of the master branch should know what they are dealing with.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>