[avalanche] support MSG_AVA_PROOF in CInv::GetCommand

Summary:
I missed this in D9360 when I introduced the MSG_AVA_PROOF inventory type.

CInv::GetCommand is used in CInv::ToString which is used in net_processing to log that the we received inv for a specific type.

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9428