codegen: move G_DEFINE_INTERFACE{,_WITH_CODE} before _default_init

codegen: move G_DEFINE_INTERFACE{,_WITH_CODE} before _default_init

to avoid warnings when built with -Wredundant-decls:

  sessionmanager-presence-generated.c:316:1: warning: redundant redeclaration of ‘session_manager_presence_default_init’ [-Wredundant-decls]
  sessionmanager-presence-generated.c:281:1: note: previous definition of ‘session_manager_presence_default_init’ was here
  sessionmanager-presence-generated.c:1273:1: warning: redundant redeclaration of ‘object_default_init’ [-Wredundant-decls]
  sessionmanager-presence-generated.c:1259:1: note: previous definition of ‘object_default_init’ was here

https://bugzilla.gnome.org/show_bug.cgi?id=696108