C front-end: fix whitespace in built-in declarations

[tautschnig]

find_pattern uses '(' (note the parenthesis) to do string look-ups of declarations of built-ins. This failed for the ones addressed in this commit as the forward declaration had a whitespace before the opening parenthesis.

• 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

Patch and project coverage have no change.

Comparison is base (ed48947) 78.50% compared to head (68ffb49) 78.50%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7587   +/-   ##
========================================
  Coverage    78.50%   78.50%           
========================================
  Files         1670     1670           
  Lines       191769   191769           
========================================
  Hits        150551   150551           
  Misses       41218    41218           

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