trivial: Fix a tiny memory leak on startup
trivial: Fix a tiny memory leak on startup

The property value is a floating reference, and so if we're not using it as a
parameter to g_variant_new() we need to unref it manually.

vathpela/fwupd,fwupd/fwupd,vathpela/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,vathpela/fwupd,vathpela/fwupd,fwupd/fwupd