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/testing.c

@@ -1201,7 +1201,7 @@ f_test_null_blob(typval_T *argvars UNUSED, typval_T *rettv)
     rettv->vval.v_blob = NULL;
 }
 
-#ifdef FEAT_JOB_CHANNEL
+#if defined(FEAT_JOB_CHANNEL) || defined(PROTO)
     void
 f_test_null_channel(typval_T *argvars UNUSED, typval_T *rettv)
 {
@@ -1216,7 +1216,7 @@ f_test_null_dict(typval_T *argvars UNUSED, typval_T *rettv)
     rettv_dict_set(rettv, NULL);
 }
 
-#ifdef FEAT_JOB_CHANNEL
+#if defined(FEAT_JOB_CHANNEL) || defined(PROTO)
     void
 f_test_null_job(typval_T *argvars UNUSED, typval_T *rettv)
 {