meson: use subproject for internal libfdt

Recent dtc/libfdt can use either Make or meson as the build system.
By using a subproject, our own meson.build can remove the hard
coded list of source files.

This is also the first step towards managing downloads with .wrap
files instead of submodule.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>