missing include

Former-commit-id: 64a94d039439785dd9a7aa857cc2d110b6957bbc

include/igl/ears.cpp

@@ -1,6 +1,7 @@
 #include "ears.h"
 #include "on_boundary.h"
 #include "find.h"
+#include "slice.h"
 #include "mat_min.h"
 #include <cassert>