vhost_user: fix mismatched lifetime warning

warning: hiding a lifetime that's elided elsewhere is confusing
   --> vhost/src/vhost_user/frontend.rs:131:13
    |
131 |     fn node(&self) -> MutexGuard<FrontendInternal> {
    |             ^^^^^     ---------------------------- the same lifetime is hidden here
    |             |
    |             the lifetime is elided here
    |
    = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
    = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
    |
131 |     fn node(&self) -> MutexGuard<'_, FrontendInternal> {
    |                                  +++

Signed-off-by: Alyssa Ross <hi@alyssa.is>

vhost/src/vhost_user/frontend.rs

@@ -128,7 +128,7 @@ impl Frontend {
         }
     }
 
-    fn node(&self) -> MutexGuard<FrontendInternal> {
+    fn node(&self) -> MutexGuard<'_, FrontendInternal> {
         self.node.lock().unwrap()
     }