xfce4-mixer: add missing dep on libunique

checking for unique-1.0 >= 1.1... not found
*** The required package unique-1.0 was not found on your system.
*** Please install unique-1.0 (atleast version 1.1) or adjust
*** the PKG_CONFIG_PATH environment variable if you
*** installed the package in a nonstandard prefix so that
*** pkg-config is able to find it.
Configure failed. The contents of all config.log files follows to aid debugging

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>