Fix type warning on certain compilers (#1193)

repetition_indices is populated with size_t values, so typing it
accordingly.

src/benchmark.cc

diff --git a/src/benchmark.cc b/src/benchmark.cc
index f9007e7..ccecfbd 100644
--- a/src/benchmark.cc
+++ b/src/benchmark.cc
@@ -329,7 +329,7 @@
     }
     assert(runners.size() == benchmarks.size() && "Unexpected runner count.");
 
-    std::vector<int> repetition_indices;
+    std::vector<size_t> repetition_indices;
     repetition_indices.reserve(num_repetitions_total);
     for (size_t runner_index = 0, num_runners = runners.size();
          runner_index != num_runners; ++runner_index) {