whitespaces nits in CMakeLists

Summary:
This has been bugging me every time I update these files. My editor
automatically strips trailing whitespace throughout the file, and appends
a trailing newline. I've gone through all the CMakeLists and found all
the nits.

Test Plan: run cmake

Reviewers: deadalnix, Fabien, jasonbcox, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3590