Explicitly enable the std feature for future-proofing

Author: dequbed

Cargo.toml

@@ -26,7 +26,7 @@ regex = "1.0"
 bufstream = "0.1.3"
 imap-proto = "0.16.1"
 nom = { version = "7.1.0", default-features = false }
-rsasl = { version = "2.0.0-rc.2", default-features = false, features = ["provider_base64"], optional = true }
+rsasl = { version = "2.0.0-rc.2", default-features = false, features = ["std", "provider_base64"], optional = true }
 base64 = "0.13.0"
 chrono = { version = "0.4", default-features = false, features = ["std"]}
 lazy_static = "1.4"