G_VARIANT_BUILDER_INIT() was only added in glib 2.50, hence we cannot use
it.

Maybe nm-glib.h should provide a compat macro, but the macro relies
on the magic number GVSB_MAGIC_PARTIAL, which is private to glib.
It's not clear that we can savely provide such a compat implementation
for older glib variants.

Fixes: 642f15f2