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: ff69c21a85 ("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>

tools/bpf/bpftool/Documentation/Makefile

@@ -23,7 +23,12 @@ DOC_MAN8 = $(addprefix $(OUTPUT),$(_DOC_MAN8))
 man: man8
 man8: $(DOC_MAN8)
 
+RST2MAN_DEP := $(shell command -v rst2man 2>/dev/null)
+
 $(OUTPUT)%.8: %.rst
+ifndef RST2MAN_DEP
+	$(error "rst2man not found, but required to generate man pages")
+endif
 	$(QUIET_GEN)rst2man $< > $@
 
 clean: