docs: sphinx/kfigure.py: Delegate inkscape msg to kernellog.verbose

docs: sphinx/kfigure.py: Delegate inkscape msg to kernellog.verbose

Depending on its version, distro config, and system-setup type,
inkscape(1) emits various warning messages which are harmless in
command-line uses.

List of such warning messages (incomplete, long ones wrapped):

  - Gtk-Message: hh:mm:ss.nnn: Failed to load module "canberra-gtk-module"
  - Unable to init server: Could not connect: Connection refused
  - Failed to get connection
  - ** (inkscape:xxx): CRITICAL **: hh:mm:ss.nnn: dbus_g_proxy_new_for_name:
    assertion 'connection != NULL' failed
  - ** (inkscape:xxx): CRITICAL **: hh:mm:ss.nnn: dbus_g_proxy_call:
    assertion 'DBUS_IS_G_PROXY (proxy)' failed
  - ** (inkscape:xxx): CRITICAL **: hh:mm:ss.nnn: dbus_g_connection_register_g_object:
    assertion 'connection != NULL' failed
  - ** (inkscape:xxx): WARNING **: hh:mm:ss.nnn:
    Fonts dir '/usr/share/inkscape/fonts' does not exist and will be ignored.

To avoid unnecessary anxiety, capture the message and output it via
kernellog.verbose or kernellog.warn depending on the exit code.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/e26a7b53-9155-8394-4a31-6006379b65a5@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>