Restore missing header <ctime> causing Visual Studio build to fail.

During a recent refactoring (450845d) a header was removed from
symex_coverage.cpp that is now causing a Build Error when building
under Visual Studio on Windows.

Adding the header back eliminates this build error and the build
succeeds after that.

Author: NlightNFotis

src/goto-checker/symex_coverage.cpp

@@ -14,6 +14,7 @@ Date: March 2016
 #include "symex_coverage.h"
 
 #include <chrono>
+#include <ctime>
 #include <fstream>
 #include <iostream>