Fix clippy in benchmarks

meithecatte · meithecatte · commit 7514c8f87c0f · 2022-03-14T12:57:05.000+01:00
I have no I idea why this worked when I ran `cargo bench`, but also I
don't really care.
diff --git a/benchmarks/Cargo.toml b/benchmarks/Cargo.toml
@@ -2,6 +2,7 @@
 name = "enumflags-benchmarks"
 version = "0.0.0"
 publish = false
+edition = "2018"
 
 [dependencies.enumflags2]
 path = "../"
