selftests/bpf: Prevent runqslower from racing on building bpftool

runqslower's Makefile is building/installing bpftool into
$(OUTPUT)/sbin/bpftool, which coincides with $(DEFAULT_BPFTOOL). In practice
this means that often when building selftests from scratch (after `make
clean`), selftests are racing with runqslower to simultaneously build bpftool
and one of the two processes fail due to file being busy. Prevent this race by
explicitly order-depending on $(BPFTOOL_DEFAULT).

Fixes: a2c9652f

 ("selftests: Refactor build to remove tools/lib/bpf from include path")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200805004757.2960750-1-andriin@fb.com