Kernel: Add operator delete for KString

This doesn't change anything because our global operator delete also
calls kfree() - however instead of relying on this implementation
detail this makes this dependency more explicit.

Kernel/KString.cpp

@@ -57,4 +57,9 @@ OwnPtr<KString> KString::try_clone() const
     return try_create(view());
 }
 
+void KString::operator delete(void* string)
+{
+    kfree(string);
+}
+
 }