[Exim] Minor format string issue in Exim 4.30

There is a minor format string issue in Exim 4.30, which is fixed by
the following patch.

--- orig/src/dbfn.c
+++ mod/src/dbfn.c
@@ -211,7 +211,8 @@
         buffer));
     else
       DEBUG(D_hints_lookup)
-        debug_printf(CS string_open_failed(save_errno, "DB file %s\n", buffer));
+        debug_printf("%s", CS string_open_failed(save_errno, "DB file %s\n",
+                                                 buffer));
     }
   close(dbblock->lockfd);
   errno = save_errno;