xfce4-weather-plugin: add libsoup-2.4 to DEPENDS

fixes (detected in Martin's) world build:

| configure:14617: checking for libsoup-2.4 >= 2.26.0
| configure:14659: result: not found

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>