fix bitflags dependency

Cargo.toml

@@ -23,3 +23,6 @@ path = "src/demo/main.rs"
 git = "https://github.com/AngryLawyer/rust-sdl2"
 #version = "~0.0.3"
 
+[dependencies]
+
+bitflags = "*"

src/sdl2_mixer/lib.rs

@@ -5,6 +5,8 @@ A binding for SDL2_mixer.
 #![crate_name = "sdl2_mixer"]
 #![crate_type = "lib"]
 
+#[macro_use]
+extern crate bitflags;
 extern crate libc;
 extern crate sdl2;