Set whoami default-features to false

Otherwise, whoami pulls in web-sys, wasm-bindgen and a BUNCH of additional dependencies. This is really unnecessary, and if someone has an actual use case where they are attempting to connect to postgres from a browser, well ... they've probably already been pwned by now. If it is deemed necessary, then add an additional activation feature for that specific slew of deps.

sqlx-core/Cargo.toml

@@ -164,7 +164,7 @@ smallvec = "1.7.0"
 url = { version = "2.2.2", default-features = false }
 uuid = { version = "1.0", default-features = false, optional = true, features = ["std"] }
 webpki-roots = { version = "0.22.0", optional = true }
-whoami = { version = "1.2.1", optional = true }
+whoami = { version = "1.2.1", default-features = false, optional = true }
 stringprep = "0.1.2"
 bstr = { version = "0.2.17", default-features = false, features = ["std"], optional = true }
 git2 = { version = "0.14", default-features = false, optional = true }