Update for libsoup-3.0

I have libsoup-3.2.2 installed for the new Gnome stuff, and noticed that xfce4-screenshooter cannot use it:

checking pkg-config is at least version 0.9.0... yes
checking for libsoup-2.4 >= 2.26.0... not found
*** The required package libsoup-2.4 was not found on your system.
*** Please install libsoup-2.4 (atleast version 2.26.0) 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.

This isn't urgent since libsoup-2.4 can be installed alongside v3.0, but I figured it's worth putting on the TODO list.