Fix derivation of Serialize and Deserialize

* Bitflags 2.0.0 now uses a newtype, so we want serde(transparent) to
  actually (de-)serialize the field of that newtype.
* We need to specify serde as a feature for Bitflags in Cargo.toml.

Cargo.toml

@@ -23,3 +23,6 @@ e2p-sys = { version= "0.1.0", git = "https://github.com/michaellass/e2p-sys.git"
 bitflags = "^2.0.0"
 e2p-sys = { version= "0.1.0", git = "https://github.com/michaellass/e2p-sys.git" }
 serde = { version = "^1.0.115", optional = true, features = ["derive"] }
+
+[features]
+serde = ["dep:serde", "bitflags/serde"]

src/lib.rs

@@ -38,7 +38,7 @@ use std::path::Path;
 
 bitflags! {
     #[derive(Copy, Clone, Debug, Default, Eq, Hash, PartialEq)]
-    #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+    #[cfg_attr(feature = "serde", derive(Serialize, Deserialize), serde(transparent))]
     pub struct Flags: u32 {
         const SECRM = EXT2_SECRM_FL;
         const UNRM = EXT2_UNRM_FL;