polkadot-node-core-pvf-common: Fix test compilation error (#5310)

This crate only uses `tempfile` on linux but includes it unconditionally
in its `Cargo.toml`. It also sets `#![deny(unused_crate_dependencies)]`.
This leads to an hard error to anything that is not Linux.

This PR fixes this error. I am wondering why CI didn't catch that.
Shouldn't the test at least be compiled (but not run) on macOS?

Author: athei

polkadot/node/core/pvf/common/Cargo.toml

@@ -42,6 +42,8 @@ seccompiler = "0.4.0"
 
 [dev-dependencies]
 assert_matches = { workspace = true }
+
+[target.'cfg(target_os = "linux")'.dev-dependencies]
 tempfile = { workspace = true }
 
 [features]