Fix rust-1.28 feature name (should be rust_1.28).

Cargo.toml

@@ -49,7 +49,7 @@ default = [
 ]
 
 # experimental; do not use; no guarantees provided that this feature will be kept
-"rust-1.28" = []
+"rust-1_28" = []
 
 [[example]]
 name = "packet2pcap"

src/phy/loopback.rs

@@ -4,9 +4,9 @@ use std::vec::Vec;
 use std::collections::VecDeque;
 #[cfg(feature = "alloc")]
 use alloc::vec::Vec;
-#[cfg(all(feature = "alloc", not(feature = "rust-1.28")))]
+#[cfg(all(feature = "alloc", not(feature = "rust-1_28")))]
 use alloc::collections::VecDeque;
-#[cfg(all(feature = "alloc", feature = "rust-1.28"))]
+#[cfg(all(feature = "alloc", feature = "rust-1_28"))]
 use alloc::VecDeque;
 
 use Result;