Placeholder icon missing when using other icon sets

[danirabbit]

What Happened?

When changing the system icon set to something other than elementary, the fallback icon we use for desktops with wireless peripherals is missing

Steps to Reproduce

1. Have a desktop with no main battery
2. Connect wireless peripherals that report battery status
3. Change the icon set for the panel to Adwaitia
4. Notice that the placeholder icon is missing

Expected Behavior

We should gresource this icon here since it's only used here

OS Version

7.x (Horus)

Software Version

Latest release (I have run all updates)

Log Output

No response

Hardware Info

No response