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_win32.pro b/src/proto/os_win32.pro
index ca5bad6..6240288 100644
--- a/src/proto/os_win32.pro
+++ b/src/proto/os_win32.pro
@@ -1,5 +1,6 @@
 /* os_win32.c */
 HINSTANCE vimLoadLib(char *name);
+int mch_is_gui_executable(void);
 HINSTANCE find_imported_module_by_funcname(HINSTANCE hInst, const char *funcname);
 void *get_dll_import_func(HINSTANCE hInst, const char *funcname);
 int dyn_libintl_init(void);