cerbero-uninstalled.ps1: Accept path with backslash
PowerShell auto-completion will fill path string with backslash.
Convert it to forward slash to make msys2 happy.
An example command line to fix is:
.\cerbero-uninstalled -c .\config\win32.cbc package gstreamer-1.0
where ".\config\win32.cbc" results from PowerShell auto-completion