Add Requires.private to pkg-config file
Add Requires.private to pkg-config file

Requires.private should be used for packages that are required to build
but which are not exposed to other applications.

http://people.freedesktop.org/~dbn/pkg-config-guide.html

plotters/accounts-sso.libsignon-glib,plotters/accounts-sso.libsignon-glib,plotters/accounts-sso.libsignon-glib