Added unsafe line in clipboard_windows.c.v

[max00217]

clipboard: Add unsafe line for windows

This PR modifies the WndClassEx struct initialization to handle lpsz_menu_name and h_icon_sm in an unsafe block. This is necessary because V language requires explicit unsafe block for operations that could lead to undefined behaviors.

Before:

// v/vlib/clipboard/clipboard_windows.c.v
.
.
.
wndclass := WndClassEx{
    cb_size: sizeof(WndClassEx)
    lpfn_wnd_proc: voidptr(&C.DefWindowProc)
    lpsz_class_name: class_name.to_wide()
    lpsz_menu_name: 0
    h_icon_sm: 0
}

After:

// v/vlib/clipboard/clipboard_windows.c.v
.
.
.
wndclass := WndClassEx{
    cb_size: sizeof(WndClassEx)
    lpfn_wnd_proc: voidptr(&C.DefWindowProc)
    lpsz_class_name: class_name.to_wide()
    lpsz_menu_name: unsafe { 0 }
    h_icon_sm: unsafe { 0 }
}

Fix this notice

C:/Program Files/v/vlib/clipboard/clipboard_windows.c.v:96:3: notice: assigning `0` to a reference field is only allowed in `unsafe` blocks
   94 |         lpfn_wnd_proc: voidptr(&C.DefWindowProc)
   95 |         lpsz_class_name: class_name.to_wide()
   96 |         lpsz_menu_name: 0
      |         ~~~~~~~~~~~~~~~~~
   97 |         h_icon_sm: 0
   98 |     }
C:/Program Files/v/vlib/clipboard/clipboard_windows.c.v:97:3: notice: assigning `0` to a reference field is only allowed in `unsafe` blocks
   95 |         lpsz_class_name: class_name.to_wide()
   96 |         lpsz_menu_name: 0
   97 |         h_icon_sm: 0
      |         ~~~~~~~~~~~~
   98 |     }
   99 |     if C.RegisterClassEx(voidptr(&wndclass)) == 0

[spytheman]

Good work.