Unify libgsd-profile and libgsd-session into installed shared library

Currently plugins can call back into the main binary; if the program
is linked with -Bsymbolic then what happens is the plugin process will
contain duplicate functions from libgsd-session.la.

Ubuntu has patched their toolchain to link with -Bsymbolic-functions
by default which causes a failure here.

This is also just cleaner and better; there's only one copy of the
code on disk.

https://bugzilla.gnome.org/show_bug.cgi?id=670244