Name type to regain macOS 10.14.6 compatibility

[tautschnig]

Avoid (spurious) ambiguity by explicitly naming the type to be constructed. This seems to be required by the compiler shipped with macOS 10.14.6.

Fixes: #7464

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• 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.
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[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change

Comparison is base (014188e) 78.50% compared to head (3174549) 78.50%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7580   +/-   ##
========================================
  Coverage    78.50%   78.50%           
========================================
  Files         1670     1670           
  Lines       191767   191768    +1     
========================================
+ Hits        150548   150549    +1     
  Misses       41219    41219           

Impacted Files	Coverage Δ
...lvers/smt2_incremental/smt_response_validation.cpp	95.38% <100.00%> (+0.02%)	⬆️

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