Import hex_literal into runtime only for benchmarks. (#6502)

substrate/bin/node/runtime/Cargo.toml

@@ -18,7 +18,7 @@ codec = { package = "parity-scale-codec", version = "1.3.1", default-features =
 integer-sqrt = { version = "0.1.2" }
 serde = { version = "1.0.102", optional = true }
 static_assertions = "1.1.0"
-hex-literal = "0.2.1"
+hex-literal = { version = "0.2.1", optional = true }
 
 # primitives
 sp-authority-discovery = { version = "2.0.0-rc3", default-features = false, path = "../../../primitives/authority-discovery" }
@@ -168,4 +168,5 @@ runtime-benchmarks = [
 	"pallet-offences-benchmarking",
 	"pallet-session-benchmarking",
 	"frame-system-benchmarking",
+	"hex-literal",
 ]