Re-add optional std feature dependencies to fix BP builds

We previously removed these, leading to failure to build under default
features including `std`.

Author: tnull

lightning-background-processor/Cargo.toml

@@ -15,7 +15,7 @@ rustdoc-args = ["--cfg", "docsrs"]
 
 [features]
 futures = [ ]
-std = []
+std = ["lightning/std", "lightning-rapid-gossip-sync/std"]
 
 default = ["std"]