Minor update to get window compilation pass.

Former-commit-id: 833421d34a86b64902dd7b348229f4fa85f655b4

include/igl/flip_edge.h

@@ -11,6 +11,7 @@
 
 #include "igl_inline.h"
 #include <Eigen/Core>
+#include <vector>
 
 namespace igl
 {