core: ignore findbugs error for not overriding hashCode()

Findbugs complains that recently added classes which override equals()
method don't override hashCode() method, thus violating the invariant
that equal objects must have equal hashcodes.

This error is irrelevant because the hash code of those classes will not
be used for sure.

Change-Id: I34b5932eddb3a54391eeab4b203d8f68ca5fe546
Signed-off-by: Arik Hadas <ahadas@redhat.com>