os2messagebox: use proper UCHAR* typecast in assignment, just in case.

src/video/os2/SDL_os2messagebox.c

@@ -377,8 +377,8 @@ static HWND _makeDlg(const SDL_MessageBoxData *messageboxdata)
 
     pDlgItem->cchText = 3; /* 0xFF, low byte of the icon Id, high byte of icon Id. */
     pDlgItem->offText = pcDlgData - (PCHAR)pTemplate;   /* Offset to the Id. */
-    /* Write susyem icon ID into dialog template. */
-    *pcDlgData = 0xFF; /* First byte is 0xFF - next 2 bytes is system pointer Id. */
+    /* Write system icon ID into dialog template. */
+    *((PBYTE)pcDlgData) = 0xFF; /* First byte is 0xFF, next 2 are system pointer Id. */
     pcDlgData++;
     *((PUSHORT)pcDlgData) = ((messageboxdata->flags & SDL_MESSAGEBOX_ERROR) != 0)?
                               SPTR_ICONERROR :