Fix getrandom build error for wasm examples

[lucky-bai]

Fixes error when building the candle-wasm-examples/whisper example (and possibly other WASM examples) with sh build-lib.sh. Currently the build fails with error:

bai@Bais-MacBook-Pro whisper % sh build-lib.sh
   Compiling getrandom v0.3.3
error: The wasm32-unknown-unknown targets are not supported by default; you may need to enable the "wasm_js" configuration flag. Note that enabling the `wasm_js` feature flag alone is insufficient. For more information see: https://docs.rs/getrandom/0.3.3/#webassembly-support

The root cause is that getrandom requires explicit configuration to select the backend when targeting wasm32-unknown-unknown.

[greenrazer]

Thanks!