MSVC compile fix for ellipse approximation

Change-Id: Ib67a2219ee171770f2ab96dcc261a610d515b6a6

examples/ellipse_approximation.cc

diff --git a/examples/ellipse_approximation.cc b/examples/ellipse_approximation.cc
index 7b2bf12..7fd8906 100644
--- a/examples/ellipse_approximation.cc
+++ b/examples/ellipse_approximation.cc
@@ -35,6 +35,7 @@
 // dynamic_sparsity, and how it can benefit problems which are numerically
 // dense but dynamically sparse.
 
+#define _USE_MATH_DEFINES
 #include <cmath>
 #include <vector>
 #include "ceres/ceres.h"