meson: Don't make object files for dtrace on macOS

meson: Don't make object files for dtrace on macOS

dtrace on macOS uses unresolved symbols with a special prefix to define
probes [1], only headers should be generated for USDT (dtrace(1)). But
it doesn't support backwards compatible no-op -G flag [2] and implicit
build rules fail.

1. https://markmail.org/message/6grq2ygr5nwdwsnb
2. https://markmail.org/message/5xrxt2w5m42nojkz

Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Cameron Esfahani <dirty@apple.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>