LibWeb: Add missing include to WebIDL::Promise

WebIDL::Promise is aliased to a JS::PromiseCapability. This missing
include would cause a compile error in an upcoming commit.

Userland/Libraries/LibWeb/WebIDL/Promise.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <LibJS/Forward.h>
+#include <LibJS/Runtime/PromiseCapability.h>
 #include <LibJS/Runtime/Value.h>
 #include <LibJS/SafeFunction.h>
 #include <LibWeb/Forward.h>