Fix the symtab2gb dependency on ansi-c

[thomasspriggs]

This directory dependency should have been added when the dependency on
the ansi-c library was added to the symtab2gb makefile. This currently
missing dependency results in sporadic build failures.

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• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
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[martin-cs]

Sorry; my fault. Thanks for catching this.

[codecov]

Codecov Report

Merging #6264 (a528058) into develop (3a2de48) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #6264   +/-   ##
========================================
  Coverage    76.16%   76.16%           
========================================
  Files         1484     1484           
  Lines       162164   162164           
========================================
  Hits        123516   123516           
  Misses       38648    38648           

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[martin-cs]

Does CMake handle this kind of dependency automatically?