largefile: fix typo that regressed large file support

With no configure.ac-early, and two configure.ac sections, gnulib-tool
only picked up the second (empty) section, and thus completely skipped
any AC_SYS_LARGEFILE call altogether.

* modules/largefile (configure.ac-early): Fix section name.

Signed-off-by: Eric Blake <eblake@redhat.com>