libsoup: Remove redundant dependency on gettext

libsoup doesn't use intl stuff directly, but uses gi18n.  libglib2
already has all required dependencies on gettext.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>