specify the features that the actor-watermark example requires

this way it won't be compiled by `cargo b --examples`
it'll be compiled by 'cargo b --examples --features actor-watermark`

Cargo.toml

@@ -19,6 +19,10 @@ version = "1.1.3"
 [lib]
 name = "rtic"
 
+[[example]]
+name = "actor-watermark"
+required-features = ["memory-watermark"]
+
 [dependencies]
 cortex-m = "0.7.0"
 cortex-m-rtic-macros = { path = "macros", version = "1.1.5" }