examples : add HEAPU8 to exported runtime methods

This commit adds `HEAPU8` to the list of exported methods.

The motivation for this commit is that currently this is causing an
error on Window systems where HEAPU8 in undefined, which results in the
following error message in the web console:
```console
main.js:1 Uncaught TypeError:
Cannot read properties of undefined (reading 'buffer') at __emval_get_property
(main.js:1:1363125) at 003a453a:0xc4a47 at 003a453a:0xc51cd at
Object.full_default (eval at craftInvokerFunction (main.js:1:1347011),
<anonymous>:9:10) at whisper.cpp/:647:42
```

Resolves: #3059

Author: danbev

examples/whisper.wasm/CMakeLists.txt

@@ -36,7 +36,7 @@ set_target_properties(${TARGET} PROPERTIES LINK_FLAGS " \
     -s MAXIMUM_MEMORY=2000MB \
     -s ALLOW_MEMORY_GROWTH=1 \
     -s FORCE_FILESYSTEM=1 \
-    -s EXPORTED_RUNTIME_METHODS=\"['print', 'printErr', 'ccall', 'cwrap']\" \
+    -s EXPORTED_RUNTIME_METHODS=\"['print', 'printErr', 'ccall', 'cwrap', 'HEAPU8']\" \
     ${EXTRA_FLAGS} \
     ")