imgui: Upgrade parking_lot to 0.12 with windows-rs bindings

`parking_lot` now uses windows-rs's `windows-sys` crate, officially
developed and vetted by Microsoft - the bindings are autogenerated from
metadata and faster to compile too.

See also Amanieu/parking_lot#311.

imgui/Cargo.toml

@@ -19,7 +19,7 @@ features = ["freetype", "docking", "tables-api"]
 bitflags = "1"
 imgui-sys = { version = "0.8.1-alpha.0", path = "../imgui-sys" }
 mint = "0.5.6"
-parking_lot = "0.11"
+parking_lot = "0.12"
 cfg-if = "1"
 
 [features]