selftests: futex: Correctly include headers dirs

When building selftests, the build system will install uapi linux
headers at usr/include in kernel source's root directory. When building
with a different output folder, the headers will be installed at
kselftests/usr/include.

Add both paths so we can build the tests using up-to-date headers.

Currently, this is uncommon to happen since it's rare to find a
build system with an outdated futex header, but it happens
when testing new futex operations.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210427135328.11013-2-andrealmeid@collabora.com