bug#38064: Simple Scan: Scanner is not detected

[sirgazil]

I can't use the scanner because Simple Scan (a.k.a. Document Scanner) does not 
detect it.


## Steps to reproduce

1. guix install simple-scan
2. Connect the scanner.
3. Launch Simple Scan.
4. Click the Scan button in Simple Scan.


## Unexpected behavior

In step 3, Simple scan displays the following warning:

```
No scanners detected

Please check your scanner is connected and powered on
```

In step 4, you see the following message:

```
Failed to scan

No scanners available. Please connect a scanner.
```


## Expected behavior

* The scanner is detected in step 3 (no warning message is shown).
* Performing step 4 makes the scanner scan.


## System information

I'm using the Guix System and simple-scan 3.34.1:

```
$ LANG=C guix describe
Generation 5    Nov 04 2019 14:24:15    (current)
  guix bf7b08c
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: bf7b08c4fe7d14c25a83bde99f19eca1119d88ff
```


## Additional information

The scanner works. I can use it from a Trisquel 8 Live USB stick, which comes 
with Simple Scan.

Running simple-scan from a terminal displays the following warnings:

```
$ LANG=C simple-scan

(simple-scan:3953): Gtk-WARNING **: 18:42:44.193: Failed to register client: 
GDBus.Error:org.gnome.SessionManager.AlreadyRegistered: Unable to register 
client
```

---
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