Makefile: Add qemu-doc.txt to distclean and dependencies

Makefile: Add qemu-doc.txt to distclean and dependencies

When qemu-doc.txt was added as a new output format in
commit f8bab10b4c3f, it was not added to either the
list of files to remove in distclean or to the dependency
line that forces qemu-options.texi to be built before
attempting to build qemu-doc.*.

In particular, the missing dependency meant that on
some platforms (notably OSX hosts) we would try to
build qemu-doc.txt before qemu-options.texi had been
fully written out, and then makeinfo would complain
about missing cross-reference targets.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1485266538-10119-1-git-send-email-peter.maydell@linaro.org
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>