tools: bpftool: exit doc Makefile early if rst2man is not available

If rst2man is not available on the system, running `make doc` from the
bpftool directory fails with an error message. However, it creates empty
manual pages (.8 files in this case). A subsequent call to `make
doc-install` would then succeed and install those empty man pages on the
system.

To prevent this, raise a Makefile error and exit immediately if rst2man
is not available before generating the pages from the rst documentation.

Fixes: ff69c21a ("tools: bpftool: add documentation")
Reported-by: Jason van Aaardt <jason.vanaardt@netronome.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>