maint: avoid double semicolon syntax check failure

maint: avoid double semicolon syntax check failure

A syntax-check recently added to gnulib would trigger a failure
(once gnulib gets updated here) for a statement introduced with
commit v8.23-43-gaf2a4ed:

  src/dd.c:806:  char const *time_fmt = _(", %g s, %s/s\n");;
  maint.mk: Double semicolon detected
  make: *** [sc_prohibit_double_semicolon] Error 1

* src/dd.c (print_xfer_stats): s/;;/;/