xinetd: follow the convention for patch location

Following 5538e4766201aa0 ("rework patch model"), the xinetd patches
were no longer being applied, because they were stored in
package/xinetd/xinetd-2.3.15/*.patch. This lead to xinetd build
failures such as:
http://autobuild.buildroot.org/results/1a032bca894b76facd9e7f01c3b5d370987d7fc8/build-end.log.

This patch fixes this by changing the location and name of the xinetd
patches to follow the new conventions.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>