long-options: avoid new GCC 7 warning from -Wimplicit-fallthrough
* lib/long-options.c (parse_long_options): Add a break statement
to avoid this new warning/failure:
$ CFLAGS='-O -Werror=implicit-fallthrough' ./gnulib-tool \
--create-testdir --dir=/t/x --with-tests --test long-options
../../gllib/long-options.c: In function ‘parse_long_options’:
../../gllib/long-options.c:66:12: error: this statement may \
fall through [-Werror=implicit-fallthrough]
(*usage_func) (EXIT_SUCCESS);
~^~~~~~~~~~~~~~~~~~~~~~~~~~~