Fix build dependencies of symtab2gb

[tautschnig]

Depending on the exact order of building directories, Makefile-based
builds of symtab2gb would sometimes fail as json.a had not been built
(and no rule for building it was present).

• Each commit message has a non-empty body, explaining why the change was made.
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• 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.
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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 4ca7c9b).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/126418525

[hannes-steffenhagen-diffblue]

This seems a bit sketchy. json-symtab-language.dir should already depend on langapi.dir and json.dir right? And linking.dir should already be included in goto-programs.dir?

If not, then I think those should be fixed to include these.

[tautschnig]

No, there isn't really that kind of dependency: it is perfectly safe to build, e.g., json-symtab-language, just the linking will fail. And that's really the only dependencies that are expressed here.

[codecov-io]

Codecov Report

Merging #5102 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5102   +/-   ##
========================================
  Coverage    69.69%   69.69%           
========================================
  Files         1321     1321           
  Lines       109346   109346           
========================================
  Hits         76205    76205           
  Misses       33141    33141

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