Makefile: Make all Sphinx documentation depend on the extensions

Makefile: Make all Sphinx documentation depend on the extensions

Add the Python source files of our Sphinx extensions to the
dependencies of the Sphinx manuals, so that if we edit the
extension source code the manuals get rebuilt.

Adding this dependency unconditionally means that we'll rebuild
a manual even if it happens to not use the extension whose
source file was changed, but this is simpler and less error
prone, and it's unlikely that we'll be making frequent changes
to the extensions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200306171749.10756-6-peter.maydell@linaro.org