efi_selftest: Ensure we include the object directory for generated headers

The current rules for generating -I lines for objects gives us both
-I/full/src/path/lib/efi_selftest and -Ilib/efi_selftest.  However, if
we re-sync the Kbuild logic we no longer get the latter implicitly.  We
will need to be explicit in such cases, so do so.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>