patch 9.1.1163: $MYVIMDIR is set too late

Problem:  $MYVIMDIR is set too late and not available while sourcing
          runtime files (Maxim Kim, after v9.1.1159)
Solution: Also set it when $MYVIMRC file is found

fixes: #16764
closes: #16767

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/optionstr.c b/src/optionstr.c
index 02806b6..f4daaba 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -135,7 +135,6 @@
 
 static int check_opt_strings(char_u *val, char **values, int list);
 static int opt_strings_flags(char_u *val, char **values, unsigned *flagp, int list);
-static void export_myvimdir(void);
 
 /*
  * After setting various option values: recompute variables that depend on
@@ -4817,7 +4816,7 @@
 /*
  * Export the environment variable $MYVIMDIR to the first item in runtimepath
  */
-    static void
+    void
 export_myvimdir()
 {
     int		dofree = FALSE;