build: Don’t link backend libraries with libfolks-internal.la helper
build: Don’t link backend libraries with libfolks-internal.la helper

Because libfolks-internal.la is a libtool helper library, and is never
actually installed, it gets statically linked into each backend. This
means that the GObject types in it (such as FolksSmallSet) get
duplicated between libfolks.so and each backend. This causes GObject
errors when the backends are loaded and attempt to re-register types
which already exist.

This is a partial revert of commits:
 • 150858a0237291d8269d222defcb82f54c1125cf
 • ef76b7cacb5cfdf7963e5c2a2fe3bc8c1c97838e

pwithnall/folks,GNOME/folks,pwithnall/folks,GNOME/folks,pwithnall/folks,GNOME/folks