Replace use of deprecated C Standard library headers in C++ code.

Change-Id: I9fa6630c7e4bdb2897df34d417635d8c7d8253bc
Signed-off-by: Tyler Chatow <tchatow@gmail.com>
diff --git a/aos/vision/blob/hierarchical_contour_merge.cc b/aos/vision/blob/hierarchical_contour_merge.cc
index ec617ca..c7de4de 100644
--- a/aos/vision/blob/hierarchical_contour_merge.cc
+++ b/aos/vision/blob/hierarchical_contour_merge.cc
@@ -1,6 +1,6 @@
 #include "aos/vision/blob/hierarchical_contour_merge.h"
 
-#include <math.h>
+#include <cmath>
 #include <queue>
 
 #include "aos/vision/blob/disjoint_set.h"