fix(examples): import proxy_add_header_map_value from the env module

The transform example declares the Proxy-Wasm host function with a bare
extern block. Under `lto = true`, rust-lld leaves it as an unresolved
symbol and the wasm32 build fails to link. Annotate the block with
`#[link(wasm_import_module = "env")]` so the symbol is emitted as a Wasm
import, matching how the proxy-wasm SDK declares its host calls.

Signed-off-by: Ramazan Kara <github@r-kara.de>

Author: RamazanKara

examples/transform/src/lib.rs

@@ -7,6 +7,10 @@
 
 // Import the host function for setting header map values.
 // map_type 2 = HTTP_RESPONSE_HEADERS in the Proxy-Wasm ABI.
+// `wasm_import_module = "env"` makes rust-lld emit this as a Wasm import from
+// the host's "env" module (as the proxy-wasm SDK does) rather than leaving it
+// as an unresolved symbol, which fails linking under `lto = true`.
+#[link(wasm_import_module = "env")]
 extern "C" {
     fn proxy_add_header_map_value(
         map_type: i32,