selftests: fix bpf build/test workflow regression when KBUILD_OUTPUT is set

commit 8ce72dc3 ("selftests: fix headers_install circular dependency")
broke bpf build/test workflow. When KBUILD_OUTPUT is set, bpf objects end
up in KBUILD_OUTPUT build directory instead of in ../selftests/bpf.

The following bpf workflow breaks when it can't find the test_verifier:

cd tools/testing/selftests/bpf; make; ./test_verifier;

Fix it to set OUTPUT only when it is undefined in lib.mk. It didn't need
to be set in the first place.

Fixes: 8ce72dc3 ("selftests: fix headers_install circular dependency")
Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>