Bug#1041807: geany-plugins: build-depends on libwnck-dev, and indirectly on GTK 2

[Bastian Germann]

Please just drop the libwnck-dev build dependency which used to be 
required for the devhelp component (which was disabled) but is not 
anymore. Consequently, there is no binary package that depends on 
libwnck22 and the package builds without it.