rust: do not use --generate-cstr

--generate-cstr is a good idea and generally the right thing to do,
but it is not available in Debian 12 and Ubuntu 22.04.  Work around
the absence.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/char/pl011/src/device.rs

@@ -12,6 +12,7 @@ use qemu_api::{
     bindings::{self, *},
     c_str,
     definitions::ObjectImpl,
+    device_class::TYPE_SYS_BUS_DEVICE,
 };
 
 use crate::{