lxpanel: fix build by switching to gdk-pixbuf-xlib
lxpanel: fix build by switching to gdk-pixbuf-xlib

The build has been broken since gdk-pixbuf-xlib was broken out into a
separate package in #88086.

For some reason if I just add gdk-pixbuf-xlib.dev the headers don't make it
into CFLAGS.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs