runsqslower: Support user-specified libbpf include and object paths

This adds support for specifying the libbpf include and object paths as
arguments to the runqslower Makefile, to support reusing the libbpf version
built as part of the selftests.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/157952561135.1683545.5660339645093141381.stgit@toke.dk