Fix Makefile clean target of src/crangler

[thomasspriggs]

This target was previously not cleaning up the generated source file for
the lexer or the built binary. This issue was seen on Ubuntu and
resulted in cmake errors when using make clean as part of switching
from using the make build to using the cmake build.

• 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.

[tautschnig]

Thank you!

[codecov]

Codecov Report

Merging #6601 (5aae9d0) into develop (7b6e5c5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #6601   +/-   ##
========================================
  Coverage    76.49%   76.49%           
========================================
  Files         1578     1578           
  Lines       181464   181464           
========================================
  Hits        138816   138816           
  Misses       42648    42648           

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