Add missing include of <ctime>

[thomasspriggs]

The type std::time_t is used in src/goto-checker/symex_coverage.cpp. std::time_t is defined in <ctime>, but was not previously included. This caused build failures locally, when building with Visual Studio Community 2019 Version 16.7.4.

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[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.95% 🎉

Comparison is base (3e3ba26) 77.89% compared to head (dcc5934) 78.84%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7871      +/-   ##
===========================================
+ Coverage    77.89%   78.84%   +0.95%     
===========================================
  Files         1699     1699              
  Lines       195294   195294              
===========================================
+ Hits        152128   153987    +1859     
+ Misses       43166    41307    -1859     

Files Changed	Coverage Δ
src/goto-checker/symex_coverage.cpp	97.87% <ø> (ø)

... and 64 files with indirect coverage changes

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