tools: bpftool: declare phony targets as such

In the Makefile, targets install, doc and doc-install should be added to
.PHONY. Let's fix this.

Fixes: 71bb428f

 ("tools: bpf: add bpftool")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>