examples: Define _GNU_SOURCE for more examples

examples: Define _GNU_SOURCE for more examples

Some of our examples use GNU/Linux extensions functions/variables:

1) domtop.c uses usleep(3),
2) logging.c uses getopt(3).

Put _GNU_SOURCE definition at the top of corresponding files so
that users can just grab the code and compile it (in contrast
with the rest of the code where _GNU_SOURCE is declared in
meson-config.h).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>