snap: stage libxcb-cursor0 to fix xcb plugin load failure on Ubuntu 24.04

Qt 5.15.x added libxcb-cursor0 as a hard runtime dependency of the xcb
platform plugin (libqxcb.so).  The kf5-5-110-qt-5-15-11-core22 content
snap does not bundle this library, so when the snap runs on an Ubuntu 24.04
host the dlopen() of the plugin fails with:

  qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in ""
  even though it was found.

Staging libxcb-cursor0 from the Ubuntu 22.04 archive satisfies the
dependency without changing the snap base, Qt version, or any other
dependency.  No ABI mismatch: the plugin and the staged library are
both built against the core22 (22.04) ABI.

Fixes issue #373.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

build-aux/snap/snapcraft.yaml

@@ -77,7 +77,16 @@ parts:
     after: [kde-sdk-setup]
     plugin: nil
     source: .
-    stage-packages: [ git, sqlite3, xdg-user-dirs ]
+    stage-packages:
+      - git
+      - sqlite3
+      - xdg-user-dirs
+      # libxcb-cursor0 was added as a hard dependency of the Qt5 xcb platform
+      # plugin in Qt 5.15.x.  The kf5-5-110-qt-5-15-11-core22 content snap does
+      # not bundle it, so dlopen() of libqxcb.so fails on Ubuntu 24.04 hosts
+      # with "found but could not load" (issue #373).  Staging the 22.04 build
+      # of this library satisfies the dep without changing base or Qt version.
+      - libxcb-cursor0
     build-packages:
       - git
       - libsqlite3-dev