libsoup-2.4.sym: remove nonexistent symbol

It was added accidentally at 1c71cb223b00eaa5c776229d9e0312ba852ef87a

https://bugzilla.gnome.org/show_bug.cgi?id=738003
libsoup: add a "make check" rule to verify libsoup-2.4.sym

Add a "make check" rule to verify libsoup-2.4.sym (which requires
fixing some incorrect sorting as well).