Fix resolution of wasm_bindgen in js! macro

This fixes an issue that, when a dependent crate depends on `stdweb` for
the use of the `js!` macro but does not specify the dependency
`wasm_bindgen`, attempting to build for the target
`wasm32-unknown-unknown` (or using `wasm-pack` and friends) will yield
the following error:

    error[E0433]: failed to resolve: use of undeclared type or module
    `wasm_bindgen`

stdweb-internal-macros/src/js_shim.rs

@@ -115,6 +115,7 @@ pub fn js_shim_extern_code( target: Target, code: &str, arg_count: usize, return
             let shim_name = &shim_name;
             let shim_args = &shim_args;
             quote! {
+                use ::stdweb::private::wasm_bindgen;
                 use ::stdweb::private::wasm_bindgen::prelude::*;
                 unsafe fn #shim_name( #(#shim_args),* ) #return_signature {
                     #[wasm_bindgen(inline_js = #code_string)]