selftests: add headers_install to lib.mk

selftests: add headers_install to lib.mk

If the kernel headers aren't installed we can't build all the tests.
Add a new make target rule 'khdr' in the file lib.mk to generate the
kernel headers and that gets include for every test-dir Makefile that
includes lib.mk If the testdir in turn have its own sub-dirs the
top_srcdir needs to be set to the linux-rootdir to be able to generate
the kernel headers.

Signed-off-by: Anders Roxell <[email protected]>
Reviewed-by: Fathi Boudra <[email protected]>
Signed-off-by: Shuah Khan (Samsung OSG) <[email protected]>