WASM: don't use a static_assert when an #error would suffice

No need to involve the compiler, make the preprocessor error out.

Change-Id: I37d3f241bd0879ba123fe26342aaaaff74c2fb6b
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/corelib/kernel/qcore_wasm.cpp

@@ -6,7 +6,7 @@
 #include <emscripten/val.h>
 
 #if !defined(Q_OS_WASM)
-static_assert(false, "This is a wasm-only file.");
+#error This is a wasm-only file.
 #endif // !defined(Q_OS_WASM)
 
 QT_BEGIN_NAMESPACE