rust: hpet: fix new warning

Nightly rustc complains that HPETAddrDecode has a lifetime but it is not
clearly noted that it comes from &self.  Apply the compiler's suggestion
to shut it up.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 6b3fad084f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

rust/hw/timer/hpet/src/hpet.rs

@@ -765,7 +765,7 @@ impl HPETState {
         self.rtc_irq_level.set(0);
     }
 
-    fn decode(&self, mut addr: hwaddr, size: u32) -> HPETAddrDecode {
+    fn decode(&self, mut addr: hwaddr, size: u32) -> HPETAddrDecode<'_> {
         let shift = ((addr & 4) * 8) as u32;
         let len = std::cmp::min(size * 8, 64 - shift);