crosvm: Supress unused mut warning for variable linux in run_vm

BUG=None
TEST=emerge-${BOARD} crosvm

Change-Id: Ib1e3629a7df98dd5a87955565757bb8ba96bdf87
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2825629
Tested-by: kokoro <noreply+kokoro@google.com>
Tested-by: Kuo-Hsin Yang <vovoy@chromium.org>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
Commit-Queue: Kuo-Hsin Yang <vovoy@chromium.org>
diff --git a/src/linux.rs b/src/linux.rs
index 5ee8fc0..43a496d 100644
--- a/src/linux.rs
+++ b/src/linux.rs
@@ -2479,6 +2479,7 @@
         fs_device_tubes.push(fs_device_tube);
     }
 
+    #[cfg_attr(not(feature = "direct"), allow(unused_mut))]
     let mut linux: RunnableLinuxVm<_, Vcpu, _> = Arch::build_vm(
         components,
         &cfg.serial_parameters,