[ISSUE] okular with speech-dispatcher support

[zenny <zenny@users.noreply.github.com>]

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New issue by zenny on void-packages repository

https://github.com/void-linux/void-packages/issues/31608

Description:
<!-- Don't request update of package. We have a script for that. https://alpha.de.repo.voidlinux.org/void-updates/void-updates.txt . However, a quality pull request may help. -->
### System

* xuname:  
  *output of ``xuname`` (part of xtools)*
```
$ xuname
Void 5.4.124_1 x86_64 AuthenticAMD uptodate hold rrrmFFFFF
```
* package:  
  *affected package(s) including the version*: ``xbps-query -p pkgver <pkgname>``
```
$ xbps-query -p pkgver okular
okular-21.04.2_1
```
### Expected behavior

Okular ought to detect the TTS engine under `Settings-> Configure Okular -> Accessibility -> Text-to-Speech engine` when engines like espeak, mimic or rhvoice  are installed.

```

### Actual behavior
 `Settings-> Configure Okular -> Accessibility -> Text-to-Speech engine`  is unselectable in okular.


### Steps to reproduce the behavior

1. Install okular and TTS engines:

`xbps-install -S okular espeak mimic speech-dispatcher qt5-speech python3-PyQt5-multimedia qt5-multimedia`

2. Open okular
3. Go to `Settings-> Configure Okular -> Accessibility -> Text-to-Speech engine` 
4. You would see something like below in the screenshot:

![pic-selected-210622-1048-48](https://user-images.githubusercontent.com/1653166/122894671-b0f64380-d347-11eb-93be-21f4fa13a951.png)