Fix "could not find sysrand_chunk in super"

Inspired by briansmith#918 (comment)

Cargo.toml

@@ -173,7 +173,7 @@ once_cell = { version = "1.8.0", default-features = false, features=["std"], opt
 [target.'cfg(any(target_os = "dragonfly", target_os = "freebsd", target_os = "illumos", target_os = "netbsd", target_os = "openbsd", target_os = "redox", target_os = "solaris"))'.dependencies]
 once_cell = { version = "1.8.0", default-features = false, features=["std"] }
 
-[target.'cfg(all(target_arch = "wasm32", target_vendor = "unknown", target_os = "unknown", target_env = ""))'.dependencies]
+[target.'cfg(all(feature = "wasm32-unknown-unknown", target_arch = "wasm32", target_vendor = "unknown", target_os = "unknown", target_env = ""))'.dependencies]
 web-sys = { version = "0.3.51", default-features = false, features = ["Crypto", "Window"], optional = true }
 
 [target.'cfg(target_os = "windows")'.dependencies]

src/rand.rs

@@ -246,6 +246,21 @@ mod sysrand_chunk {
     }
 }
 
+#[cfg(all(
+    not(feature = "wasm32_unknown_unknown_js"),
+    target_arch = "wasm32",
+    target_vendor = "unknown",
+    target_os = "unknown",
+    target_env = "",
+))]
+mod sysrand_chunk {
+    use crate::error;
+
+    pub fn chunk(mut dest: &mut [u8]) -> Result<usize, error::Unspecified> {
+        unimplemented!()
+    }
+}
+
 #[cfg(windows)]
 mod sysrand_chunk {
     use crate::{error, polyfill};