[build] .gitignore: add QT Creator artifacts

Summary:
These files appear on OSX if you follow the QT Creator instructions.

Backport of Bitcoin Core PR11989
https://github.com/bitcoin/bitcoin/pull/11989

Test Plan:

make check-all

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4063