patch 9.1.1662: Issues with proto files: missing or inconsistent prototypes.

Problem:  Issues with proto files: missing or inconsistent prototypes.
Solution: Update ifdefs, move typedefs, fix prototype declaration
          (Hirohito Higashi)

This change focuses on fixes and tweaks found while working on #18045 for
the proto/*.pro files.

The following fixes and tweaks have been made:

- Fixed a prototype declaration where the variable name differed from
  the function definition.
- Removed a prototype declaration without a function body.
- Fixed a problem where a prototype declaration was not created for a
  function definition enclosed in a #if directive because it lacked ||
  defined(PROTO).
- Moved typedef struct soundcb_S soundcb_T; from proto/sound.pro to
  vim.h.
- Other small tweaks.

The make proto mechanism remains unchanged.

closes: #18058

Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/os_mswin.c

@@ -108,7 +108,6 @@ int WSInitialized = FALSE; // WinSock is initialized
 #endif
 
 
-#ifndef PROTO
 /*
  * Save the instance handle of the exe/dll.
  */
@@ -117,7 +116,6 @@ SaveInst(HINSTANCE hInst)
 {
     g_hinst = hInst;
 }
-#endif
 
 #if defined(FEAT_GUI_MSWIN) || defined(PROTO)
 /*