qt: refactor: Use absolute include paths in .ui files

Summary:
Partial backport of core PR11651 (commit 5b56ec9)
https://github.com/bitcoin/bitcoin/pull/11651/commits/5b56ec969fcbedd87eb0cf43c16eb991aca300d0

Test Plan:

make check
ninja check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2967