selftests/bpf: Fix s390x vmlinux path

After commit edd4a866

 ("s390/boot: get rid of startup archive")
there is no more compressed/ subdirectory.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230129190501.1624747-8-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>