{wiki} AddressSanitizerFlags - show C++ example for __asan_default_options

[asmaloney]

Right now the AddressSanitizerFlags wiki page shows the following code for __asan_default_options:

const char *__asan_default_options() {
  return "verbosity=1:malloc_context_size=20";
}

To use it in C++, however, it needs to be:

extern "C" const char *__asan_default_options() {
  return "verbosity=1:malloc_context_size=20";
}

It a small thing, but it would save people time trying to figure out why it doesn't work.