docs: Makefile: Add dependency to $(YNL_INDEX) for targets other than htmldocs

Commit f061c9f7 ("Documentation: Document each netlink family")
added recipes for YAML -> RST conversion.
Then commit 7da8bdbf ("docs: Makefile: Fix make cleandocs by
deleting generated .rst files") made sure those converted .rst files
are cleaned by "make cleandocs".

However, they took care of htmldocs build only.

If one of other targets such as latexdocs or epubdocs is built
without building htmldocs, missing .rst files can cause additional
WARNINGs from sphinx-build as follow:

    ./Documentation/userspace-api/netlink/specs.rst:18: WARNING: undefined label: 'specs'
    ./Documentation/userspace-api/netlink/netlink-raw.rst:64: WARNING: unknown document: '../../networking/netlink_spec/rt_link'
    ./Documentation/userspace-api/netlink/netlink-raw.rst:64: WARNING: unknown document: '../../networking/netlink_spec/tc'
    ./Documentation/userspace-api/netlink/index.rst:21: WARNING: undefined label: 'specs'

Add dependency to $(YNL_INDEX) for other targets and allow any targets
to be built cleanly right after "make cleandocs".

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: stable@vger.kernel.org  # v6.7
Cc: Thorsten Blum <thorsten.blum@toblux.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Reviwed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <e876e3c8-109d-4bc8-9916-05a4bc4ee9ac@gmail.com>