Fix mismatching types for setDirtyBlockIndex

Summary:
The extern declaration and the definition don't match, triggering a -Wodr warning.
This issue was introduced in D12033.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12078