main/postmarketos-ui-gnome-mobile: fix gnome-mobile-extensions-app dep (MR 5899)

Fix the bug that prevented "pmbootstrap install" from working with
postmarketos-ui-gnome (not -mobile).

This bug has been here in pmaports for a long time, but until recent
changes in apk's resolver code, this did not fail (apk installed
gnome-extensions-app instead of gnome-mobile-extensions-app in that
scenario).

How it happened:
* postmarketos-ui-gnome has _pmb_recommends="gnome-extensions-app".
* pmbootstrap: pmb.install._install.get_recommends() works like this:
  1. Look at the packages i in _pmb_recommends of the packages that
     will be installed in "pmbootstrap install".
  2. For each package j in _pmb_recommends of i:
    2.1 Add all packages in j's _pmb_recommends to the list of
        packages to be installed.
    (2.2 Recurse by doing 1. with each package in j's _pmb_recommends.)
* In step 2:
  * pmaports doesn't have a gnome-extensions-app (it's in Alpine, a
    subpackage of gnome-shell).
  * Therefore our gnome-shell-mobile fork gets looked at, which has a
    gnome-mobile-extensions-app subpackage that provides
    gnome-extensions-app.
  * pmbootstrap looks at the _pmb_recommends of the gnome-shell-mobile
    package, which has "gnome-mobile-extensions-app" (this is wrong,
    it should be in postmarektos-ui-gnome-mobile instead and this patch
    fixes this!).
  * pmbootstrap adds the "gnome-mobile-extensions-app" to the list of
    packages to be installed.
* The result is an "apk add" line that has "postmarketos-ui-gnome" and
  "gnome-mobile-extensions-app" (and more packages), which results in a
  conflict betwen gnome and gnome-mobile packages.

Fixes: issue 3360
[ci:skip-vercheck]: _pmb_recommends changes don't need package rebuilds
[ci:skip-build]: already built successfully in CI

main/postmarketos-ui-gnome-mobile/APKBUILD

@@ -30,7 +30,7 @@ depends="
 	udisks2
 	"
 _pmb_recommends="
-	gnome-extensions-app
+	gnome-mobile-extensions-app
 	"
 subpackages="$pkgname-openrc"
 source="

temp/gnome-shell-mobile/APKBUILD

@@ -78,8 +78,6 @@ source="gnome-shell-mobile-$_commit.tar.gz::https://gitlab.gnome.org/verdre/gnom
 	gsh.patch"
 builddir="$srcdir/gnome-shell-mobile-$_commit"
 options="!check" # Tests have circular dependency 'gnome-shell <-> gdm'
-# To make sure pmb_recommends installs the correct one
-_pmb_recommends="gnome-mobile-extensions-app"
 
 prepare() {
 	default_prepare