Add missing include of <ctime>

The type `std::time_t` is used in this file. `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.

Author: thomasspriggs

src/goto-checker/symex_coverage.cpp

@@ -24,6 +24,7 @@ Date: March 2016
 #include <linking/static_lifetime_init.h>
 
 #include <chrono> // IWYU pragma: keep
+#include <ctime> // IWYU pragma: keep - For std::time_t
 #include <fstream> // IWYU pragma: keep
 #include <iostream>