patch 8.1.1230: a lot of code is shared between vim.exe and gvim.exe

Problem:    A lot of code is shared between vim.exe and gvim.exe.
Solution:   Optionally put the shared code in vim.dll. (Ken Takata,
            closes #4287)
diff --git a/src/proto/os_mswin.pro b/src/proto/os_mswin.pro
index 2548d00..6cc8a5d 100644
--- a/src/proto/os_mswin.pro
+++ b/src/proto/os_mswin.pro
@@ -1,5 +1,5 @@
 /* os_mswin.c */
-void mch_exit(int r);
+void mch_exit_g(int r);
 void mch_early_init(void);
 int mch_input_isatty(void);
 void mch_settitle(char_u *title, char_u *icon);