vim: include used definitions in headers

This is not an exhaustive commit, it merely ameliorates the situations a
bit. There are quite a few header files that don't include all the types
they use in their function/struct/... definitions. This throws of the
testing infrastructure (but is not such a problem for the main binary that
has the "tumbleweed of includes"-phenomenon).

src/nvim/normal.h

@@ -2,6 +2,7 @@
 #define NVIM_NORMAL_H
 
 #include "nvim/pos.h"
+#include "nvim/buffer_defs.h"  // for win_T
 
 /* Values for find_ident_under_cursor() */
 #define FIND_IDENT      1       /* find identifier (word) */