cargo: Add categories for discoverability (#473)

`os::android-apis` is a new category that I added upstream in
rust-lang/crates.io#8558.  The rest already
exists on crates.io, and should make it easier to find/place this
crate.

Author: MarijnS95

ndk-sys/Cargo.toml

@@ -11,6 +11,7 @@ documentation = "https://docs.rs/ndk-sys"
 homepage = "https://github.com/rust-mobile/ndk"
 repository = "https://github.com/rust-mobile/ndk"
 rust-version = "1.60"
+categories = ["os", "os::android-apis", "external-ffi-bindings"]
 
 [dependencies]
 jni-sys = "0.3.0"

ndk/Cargo.toml

@@ -11,6 +11,7 @@ documentation = "https://docs.rs/ndk"
 homepage = "https://github.com/rust-mobile/ndk"
 repository = "https://github.com/rust-mobile/ndk"
 rust-version = "1.66"
+categories = ["os", "os::android-apis", "api-bindings"]
 
 [features]
 default = ["rwh_06"]