Fix std::abs overload, required cmath header (#44)

highwayhash/robust_statistics.h

diff --git a/highwayhash/robust_statistics.h b/highwayhash/robust_statistics.h
index abfa57d..4e45494 100644
--- a/highwayhash/robust_statistics.h
+++ b/highwayhash/robust_statistics.h
@@ -20,6 +20,7 @@
 #include <stddef.h>
 #include <algorithm>
 #include <cassert>
+#include <cmath>
 #include <limits>
 #include <vector>
 
@@ -124,7 +125,7 @@
   std::vector<T> abs_deviations;
   abs_deviations.reserve(samples.size());
   for (const T sample : samples) {
-    abs_deviations.push_back(std::abs(static_cast<T>(sample - median)));
+    abs_deviations.push_back(std::abs(sample - median));
   }
   return Median(&abs_deviations);
 }