[backport#15091] Qt: Fix update headers-count

Summary:
https://github.com/bitcoin/bitcoin/pull/15091/commits/e8db6b8044424a6fbb4b0a80acffc8c7fbf6be8b

Depends on D6179

Concludes backport of Core PR15091

Test Plan:

ninja
./src/qt/bitcoin-qt

If node isnt 100% synced, check that 'Syncing headers' message appears

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6180