tools/bpftool: Force clean of out-of-tree build

Cleaning a partial build can fail if the output directory for libbpf
wasn't created:

$ make -C tools/bpf/bpftool O=/tmp/bpf clean
/bin/sh: line 0: cd: /tmp/bpf/libbpf/: No such file or directory
tools/scripts/Makefile.include:17: *** output directory "/tmp/bpf/libbpf/" does not exist.  Stop.
make: *** [Makefile:36: /tmp/bpf/libbpf/libbpf.a-clean] Error 2

As a result make never gets around to clearing the leftover objects. Add
the libbpf output directory as clean dependency to ensure clean always
succeeds (similarly to the "descend" macro). The directory is later
removed by the clean recipe.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201110164310.2600671-3-jean-philippe@linaro.org