Use "%zd" format specifier for printing number of testcases executed.

Summary:
This helps to avoid signed integer overflow after running a fast fuzz target for several hours, e.g.:

<...>
Done -1097903291 runs in 54001 second(s)



Reviewers: kcc

Reviewed By: kcc

Differential Revision: https://reviews.llvm.org/D29941

llvm-svn: 295112

llvm/lib/Fuzzer/FuzzerDriver.cpp

@@ -567,7 +567,7 @@ int FuzzerDriver(int *argc, char ***argv, UserCallback Callback) {
   F->Loop();
 
   if (Flags.verbosity)
-    Printf("Done %d runs in %zd second(s)\n", F->getTotalNumberOfRuns(),
+    Printf("Done %zd runs in %zd second(s)\n", F->getTotalNumberOfRuns(),
            F->secondsSinceProcessStartUp());
   F->PrintFinalStats();