Crangler: silence lexer build warnings

[tautschnig]

Some versions of compilers (combined with particular flex variants) will
result in compiler warnings that we already silence for various others
lexers.

• 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 #6501 (a8e14d5) into develop (e2f4fec) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #6501   +/-   ##
========================================
  Coverage    76.08%   76.08%           
========================================
  Files         1548     1548           
  Lines       166387   166387           
========================================
  Hits        126591   126591           
  Misses       39796    39796           

Impacted Files	Coverage Δ
src/crangler/scanner.l	63.63% <ø> (ø)

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