patch 9.1.0584: Warning about redeclaring f_id() non-static

Problem:  Warning about redeclaring f_id() non-static
Solution: Declare f_id() static (John Marriott)

closes: #15257

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/evalfunc.c b/src/evalfunc.c
index c092421..a5e4b20 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -7534,7 +7534,7 @@
  * Currently only valid for object/container types.
  * Return empty string if not an object.
  */
-    void
+    static void
 f_id(typval_T *argvars, typval_T *rettv)
 {
     char    numbuf[NUMBUFLEN];