[wasm][stdlib] Don't include <thread> with swift-threading-package=none

We can't assume libcxx to be built with `LIBCXX_ENABLE_THREADS=YES` with
none threading package.

Author: kateinoigakukun

stdlib/public/stubs/Stubs.cpp

@@ -58,7 +58,9 @@
 #endif // SWIFT_STDLIB_HAS_LOCALE
 
 #include <limits>
+#ifndef SWIFT_THREADING_NONE
 #include <thread>
+#endif
 
 #if defined(__ANDROID__)
 #include <android/api-level.h>