task-xfce-base: add thunar-volman

This fixes:

ERROR: There is a comment on line 34 of file /OE/tentacle/sources/meta-openembedded/meta-xfce/recipes-xfce/tasks/task-xfce-base.bb (#    thunar-volman \) which is in the middle of a multiline expression.
Bitbake used to ignore these but no longer does so, please fix your metadata as errors are likely as a result of this change.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Acked-by: Andreas Müller <schnitzeltony@gmx.de>