only run bindgen on ImPlot... names

not sure why bindgen switches to i32 everywhere
moved imgui dependency up one step as this seems where its actually used

implot-sys/src/lib.rs

@@ -2,6 +2,8 @@
 #![allow(non_camel_case_types)]
 #![allow(non_snake_case)]
 
-pub use imgui;
+// just for linking for tests
+#[cfg(test)]
+use imgui_sys as _;
 
 include!("bindings.rs");