Remove unused variable and tidy test #includes.

Change-Id: I0eaeea0d50f66302d632aab400cfa48dc301ef69
diff --git a/runtime/reflection_test.cc b/runtime/reflection_test.cc
index c14da5d..5332202 100644
--- a/runtime/reflection_test.cc
+++ b/runtime/reflection_test.cc
@@ -16,7 +16,7 @@
 
 #include "reflection.h"
 
-#include <cmath>
+#include <limits.h>
 
 #include "common_compiler_test.h"
 #include "mirror/art_method-inl.h"