Use $(KHDR_INCLUDES) as lookup path for kernel headers. This prevents
building against kernel headers from the build environment in scenarios
where kernel headers are installed into a specific output directory
(O=...).
[ Hopefully I did not break too many things with this selftests-wide
change. Additional testing would be welcome before merging, especially
given that whenever we get this wrong, it appears to fall-back on the
build environment system headers, which hides issues. This applies on
top of v6.2-rc3. ]