fix header.

Former-commit-id: ed3d12e3a339a7ea5be2a6c7acd1a4ee85056bf1

include/igl/Camera.h

@@ -16,6 +16,7 @@
 
 #include <Eigen/Geometry>
 #include <Eigen/Core>
+#include <PI.h>
 
 #define IGL_CAMERA_MIN_ANGLE 5.0
 namespace igl