patch 8.2.5125: MS-Windows: warnings from MinGW compiler

Problem:    MS-Windows: warnings from MinGW compyler.
Solution:   Use "volatile". (Yasuhiro Matsumoto, closes #10589)  Initialize
            variable.
diff --git a/src/proto/os_win32.pro b/src/proto/os_win32.pro
index 706cbd6..dd2c27e 100644
--- a/src/proto/os_win32.pro
+++ b/src/proto/os_win32.pro
@@ -85,5 +85,5 @@
 void resize_console_buf(void);
 char *GetWin32Error(void);
 void stop_timeout(void);
-const int *start_timeout(long msec);
+volatile int *start_timeout(long msec);
 /* vim: set ft=c : */