cargo: Add missing sync feature to all (#444)

The docs for this module are missing because its contents weren't picked
up in the docs.rs build.

Author: MarijnS95

ndk/Cargo.toml

@@ -14,7 +14,7 @@ rust-version = "1.66"
 
 [features]
 default = ["rwh_06"]
-all = ["audio", "bitmap","media", "api-level-31", "rwh_04", "rwh_05", "rwh_06"]
+all = ["audio", "bitmap", "media", "sync", "api-level-31", "rwh_04", "rwh_05", "rwh_06"]
 
 audio = ["ffi/audio", "api-level-26"]
 bitmap = ["ffi/bitmap"]