BenchSysTimer_mach drops all sub-ms precision. Fix that!

bench/BenchSysTimer_mach.cpp

Index: bench/BenchSysTimer_mach.cpp
diff --git a/bench/BenchSysTimer_mach.cpp b/bench/BenchSysTimer_mach.cpp
index cf3f1c1f7d408514d57fddac2aae1c4c9b2eb9a2..1f0a6b98615fc3733ea50a0b7c9357bd876253a4 100644
--- a/bench/BenchSysTimer_mach.cpp
+++ b/bench/BenchSysTimer_mach.cpp
@@ -71,6 +71,6 @@ double BenchSysTimer::endWall() {
     } else {
         uint64_t elapsedNano = elapsed * sTimebaseInfo.numer
                                / sTimebaseInfo.denom;
-        return elapsedNano / 1000000;
+        return elapsedNano / 1000000.0;
     }
 }