check-uapi: use dummy libc includes

Based on Thomas Weißschuh's series to kernel headers to no longer require
an installed libc when build testing the uapi headers, the same can now
be done for the scripts/check-uapi.sh script.

The only required change here is to add the  usr/dummy-include include
path.

Link: https://lore.kernel.org/lkml/20260227-kbuild-uapi-libc-v1-0-c17de0d19776@weissschuh.net/ [1]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>