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/gui.pro b/src/proto/gui.pro
index 4004bc7..f8f80b5 100644
--- a/src/proto/gui.pro
+++ b/src/proto/gui.pro
@@ -1,5 +1,5 @@
 /* gui.c */
-void gui_start(void);
+void gui_start(char_u *arg);
 void gui_prepare(int *argc, char **argv);
 int gui_init_check(void);
 void gui_init(void);