Add ctime include

Without the ctime, compiling with mingw and gcc 4.8.1 I get the error:
>g++ wikisort.cpp -o wikisort -O3
wikisort.cpp: In function 'double Seconds()':
wikisort.cpp:16:33: error: 'clock' was not declared in this scope
 double Seconds() { return clock() * 1.0/CLOCKS_PER_SEC; }
                                 ^
wikisort.cpp:16:41: error: 'CLOCKS_PER_SEC' was not declared in this scope
 double Seconds() { return clock() * 1.0/CLOCKS_PER_SEC; }
                                         ^
wikisort.cpp: In function 'int main()':
wikisort.cpp:639:17: error: 'time' was not declared in this scope
  srand(time(NULL));
                 ^

It now works on Windows (with mingw) and Linux.