eframe: Fix building the wasm32 docs for docs.rs (#3757)

* Closes https://github.com/emilk/egui/issues/3756

Fix suggested by @morionelab

crates/eframe/Cargo.toml

@@ -21,6 +21,7 @@ include = [
 
 [package.metadata.docs.rs]
 all-features = true
+rustc-args = ["--cfg=web_sys_unstable_apis"]
 targets = ["x86_64-unknown-linux-gnu", "wasm32-unknown-unknown"]
 
 [lib]