Fix missing native-wasm source set in legacy docs build

(cherry picked from commit 69b90e1)

Author: ilya-g

libraries/tools/kotlin-stdlib-docs-legacy/build-docs.xml

@@ -260,6 +260,7 @@
         <sourceroot path="${kotlin_native_root}/Interop/Runtime/src/native/kotlin"/>
         <sourceroot path="${kotlin_native_root}/Interop/JsRuntime/src/main/kotlin"/>
         <sourceroot path="${kotlin_native_root}/runtime/src/main/kotlin"/>
+        <sourceroot path="${kotlin_stdlib_dir}/native-wasm/src"/>
 
         <packageoptions prefix="kotlin.native.internal" suppress="true"/>
         <packageoptions prefix="kotlin.test" suppress="true"/>