main/postmarketos-base-ui-gnome: add accidently removed line

Follow-up of postmarketOS/pmaports!5824.

Accidently removed `decibels` in a MR revision:
https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/5824/diffs?diff_id=27482&start_sha=a5318d3eef5f6d08c08e086098a42af461f768fe

[ci:skip-vercheck]

main/postmarketos-base-ui-gnome/APKBUILD

@@ -31,6 +31,7 @@ options="!check"
 subpackages="$pkgname-openrc"
 
 _pmb_recommends="
+	decibels
 	firefox-esr
 	flatpak
 	gnome-calculator