Use IterationCount in one more place

Found in -UNDEBUG build
diff --git a/src/statistics.cc b/src/statistics.cc
index 4dc2c95..bd5a3d6 100644
--- a/src/statistics.cc
+++ b/src/statistics.cc
@@ -97,7 +97,7 @@
 
   // All repetitions should be run with the same number of iterations so we
   // can take this information from the first benchmark.
-  int64_t const run_iterations = reports.front().iterations;
+  const IterationCount run_iterations = reports.front().iterations;
   // create stats for user counters
   struct CounterStat {
     Counter c;