flatpak: add pulseaudio support for sound recording

video recording requires pulseaudio support. Otherwise following error
happens:

```
Phase 2: mpegize --  /home/mobian/Videos/VID20250331075934.mkv
Traceback (most recent call last):
  File "/app/libexec/megapixels//mpegize.py", line 546, in <module>
    m.run(sys.argv)
  File "/app/libexec/megapixels//mpegize.py", line 462, in run
    gst_convert(m.base, argv[3], ext=="dng")
  File "/app/libexec/megapixels//mpegize.py", line 260, in gst_convert
    t2 = audio.get_time()
         ^^^^^^^^^^^^^^^^
  File "/app/libexec/megapixels//mpegize.py", line 102, in get_time
    s = s[:-m.slen]
        ~^^^^^^^^^^
TypeError: 'NoneType' object is not subscriptable
```

flatpak/me.gapixels.Megapixels.yml

@@ -17,6 +17,9 @@ finish-args:
   # wayland access
   - --socket=wayland
 
+  # sound recording
+  - --socket=pulseaudio
+
   # Needs to save file locally
   - --filesystem=xdg-pictures
   - --filesystem=xdg-videos