testsuite: Handle KDIR=any

testsuite: Handle KDIR=any

I update my kernel frequently and may not be running a kernel with
headers installed. That always leads me to:

	1) run `meson test -C build`
	2) Oh, crap, it fails: no kernel headers
	3) Find whatever kernel header is installed and point KDIR to it

We can do better and tell KDIR=any to use whatever it finds.

Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/384