Makefile: Fix 'make distclean'

We forgot the '-r' option on the rm command to clean up the
Sphinx .doctrees working directory, which meant that
"make distclean" fails:
 rm: cannot remove '.doctrees': Is a directory

Add the missing option.

Fixes: 5f71eac0 ("Makefile, configure: Support building rST documentation")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190308135744.6480-3-peter.maydell@linaro.org