xfce4-settings: add garcon to DEPENDS

error message was:
checking for garcon-1 >= 0.1.10... not found
*** The required package garcon-1 was not found on your system.
*** Please install garcon-1 (atleast version 0.1.10) 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: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>