package/rt-tests: make numactl an optional dependency

package/rt-tests: make numactl an optional dependency

Currently, rt-tests don't show up in menuconfig and cannot be built if
BR2_PACKAGE_NUMACTL_ARCH_SUPPORTS is not available for the target
architecture. In such a case numactl is also not available what
applies for most small embedded targets.

This dependency was introduced with commit 7f50cbfb800e
("package/rt-tests: bump to version 1.8")

But rt-tests can also be build without numactl when passing NUMA=0 to make.
This possibility is documented on
https://wiki.linuxfoundation.org/realtime/documentation/howto/tools/rt-tests

So this commit changes rt-tests to handle numactl as an optional
dependency.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>