Register the json_symtab languauge in symtab2gb

[martin-cs]

To quote langapi/mode.cpp:

"Note: registering a language is required for using the functions
in language_util.h"

symtab2gb previously failed to do this and so fails in invariant
under some obscure and unclear circumstances.

• 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]

Unfortunately I don't have access to the test case that triggers this but as it is a violation of our own documentation, I figure it is clearly enough a bug.

[tautschnig]

Failing build note: symtab2gb_parse_options.cpp:114:3: error: ‘register_language’ was not declared in this scope

[martin-cs]

Ooops. Should be fixed now.

[codecov]

Codecov Report

Merging #6175 (3b5b849) into develop (6b50008) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6175      +/-   ##
===========================================
- Coverage    75.30%   75.26%   -0.04%     
===========================================
  Files         1450     1450              
  Lines       158480   158481       +1     
===========================================
- Hits        119348   119287      -61     
- Misses       39132    39194      +62     

Impacted Files	Coverage Δ
src/symtab2gb/symtab2gb_parse_options.cpp	61.29% <100.00%> (+0.63%)	⬆️
src/ansi-c/gcc_types.cpp	41.86% <0.00%> (-46.52%)	⬇️
src/ansi-c/scanner.l	40.71% <0.00%> (-1.66%)	⬇️
src/ansi-c/parser.y	56.54% <0.00%> (-1.58%)	⬇️

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[martin-cs]

I am not sure how the over-all coverage is decreasing if the patch has 100% coverage.