selftests/bpf: Add docs target as all dependency

Currently docs target is make dependency for TEST_GEN_FILES,
which makes tests to be rebuilt every time you run make.

Adding docs as all target dependency, so when running make
on top of built selftests it will show just:

  $ make
  make[1]: Nothing to be done for 'docs'.

After cleaning docs, only docs is rebuilt:

  $ make docs-clean
  CLEAN    eBPF_helpers-manpage
  CLEAN    eBPF_syscall-manpage
  $ make
  GEN      ...selftests/bpf/bpf-helpers.rst
  GEN      ...selftests/bpf/bpf-helpers.7
  GEN      ...selftests/bpf/bpf-syscall.rst
  GEN      ...selftests/bpf/bpf-syscall.2
  $ make
  make[1]: Nothing to be done for 'docs'.

Fixes: a01d935b ("tools/bpf: Remove bpf-helpers from bpftool docs")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210420132428.15710-1-jolsa@kernel.org