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/buffer.h

@@ -2,6 +2,7 @@
 #define NVIM_BUFFER_H
 
 #include "nvim/pos.h"  // for linenr_T
+#include "nvim/ex_cmds_defs.h"  // for exarg_T
 
 /* Values for buflist_getfile() */
 #define GETF_SETMARK    0x01    /* set pcmark before jumping */

src/nvim/buffer_defs.h

@@ -2,10 +2,12 @@
 #define NVIM_BUFFER_DEFS_H
 
 #include <stdbool.h>
+// for FILE
+#include <stdio.h>
 
 // for garray_T
 #include "nvim/garray.h"
-// for pos_T and lpos_T
+// for pos_T, lpos_T and linenr_T
 #include "nvim/pos.h"
 // for the number window-local and buffer-local options
 #include "nvim/option_defs.h"

src/nvim/ex_eval.h

@@ -2,6 +2,7 @@
 #define NVIM_EX_EVAL_H
 
 #include "nvim/pos.h"           // for linenr_T
+#include "nvim/ex_cmds_defs.h"  // for exarg_T
 
 /*
  * A list used for saving values of "emsg_silent".  Used by ex_try() to save the

src/nvim/message.h

@@ -2,6 +2,7 @@
 #define NVIM_MESSAGE_H
 
 #include <stdbool.h>
+#include "nvim/eval_defs.h"  // for typval_T
 
 /*
  * Types of dialogs passed to do_dialog().

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) */

src/nvim/undo_defs.h

@@ -1,6 +1,8 @@
 #ifndef NVIM_UNDO_DEFS_H
 #define NVIM_UNDO_DEFS_H
 
+#include <time.h>  // for time_t
+
 #include "nvim/pos.h"
 
 /* Structure to store info about the Visual area. */