Mark unnecessarily-public members of compilet protected

[tautschnig]

compilet has several public members, which should eventually have
setters/getters instead. To make clearer which those are, first mark
those that aren't needed outside the class protected.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5617 (be12593) into develop (4144414) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5617   +/-   ##
========================================
  Coverage    69.32%   69.32%           
========================================
  Files         1241     1241           
  Lines       100432   100432           
========================================
  Hits         69625    69625           
  Misses       30807    30807           

Flag	Coverage Δ
cproversmt2	43.08% <ø> (ø)
regression	66.22% <ø> (ø)
unit	32.27% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/goto-cc/compile.h	100.00% <ø> (ø)

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[NlightNFotis]

Reasonable.