selftests: ublk: kublk: fix include path

selftests: ublk: kublk: fix include path

Building kublk currently fails (with a "could not find linux/ublk_cmd.h"
error message) if kernel headers are not installed in a system-global
location (i.e. somewhere in the compiler's default include search path).
This failure is unnecessary, as make kselftest installs kernel headers
in the build tree - kublk's build just isn't looking for them properly.
There is an include path in kublk's CFLAGS which is probably intended to
find the kernel headers installed in the build tree; fix it so that it
can actually find them.

This introduces some macro redefinition issues between glibc-provided
headers and kernel headers; fix those by eliminating one include in
kublk.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250429-ublk_selftests-v2-3-e970b6d9e4f4@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>