refactor/single-include: ui.h, ui_bridge.h, ugrid.h (#6571)

src/nvim/ui.h

@@ -6,6 +6,7 @@
 #include <stdint.h>
 
 #include "api/private/defs.h"
+#include "nvim/buffer_defs.h"
 
 typedef struct {
   bool bold, underline, undercurl, italic, reverse;