build: Remove illegal spacing in darwin.mk

Summary:
Backport of core PR14647.

For the sake of avoiding merge conflicts.

Test Plan:
Untested (by me), I trust the core devs that it is doing something. For
what I see it can't hurt.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5602