storage: Add function to discover users that have prints

For sensors with internal storage we may want to garbage collect prints.
Adding this API means we can list all local prints, allowing us to find
out whether there are prints on the device's storage with no
corresponding print on the host.

src/file_storage.h

@@ -38,5 +38,6 @@ int file_storage_init(void);
 int file_storage_deinit(void);
 
 GSList *file_storage_discover_prints(FpDevice *dev, const char *username);
+GSList *file_storage_discover_users(void);
 
 #endif