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/proto/optionstr.pro b/src/proto/optionstr.pro
index 75a8d73..11b9d05 100644
--- a/src/proto/optionstr.pro
+++ b/src/proto/optionstr.pro
@@ -198,4 +198,5 @@
 int check_ff_value(char_u *p);
 void save_clear_shm_value(void);
 void restore_shm_value(void);
+void export_myvimdir(void);
 /* vim: set ft=c : */