Make myself codeowner for CMake files

[hannes-steffenhagen-diffblue]

Proposing myself to be added as codeowner for CMake related files, because based on observation I’m probably the person who cares the most about CMake around here.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[martin-cs]

Yes; it is good to align owner, maintainer and "care about". To avoid bottlenecks and so that you have people to approve your PRs -- is there anyone else that could be listed?

[chrisr-diffblue]

As you are our resident CMake expert :-) this makes total sense to me!

[codecov]

Codecov Report

Merging #5466 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5466   +/-   ##
========================================
  Coverage    68.25%   68.25%           
========================================
  Files         1180     1180           
  Lines        97722    97722           
========================================
  Hits         66698    66698           
  Misses       31024    31024           

Flag	Coverage Δ
#cproversmt2	42.81% <ø> (ø)
#regression	65.42% <ø> (ø)
#unit	32.23% <ø> (ø)

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