Add gtk4-layer-shell-devel to installation instructions

[SaudAlshareef]

Resolves Build Error

The build currently fails because the system library gtk4-layer-shell-0 is missing (tested only on opensuse idk if you need to update in other distros as well but keep up the good work :D):

error: error: unable to find dynamic system library 'gtk4-layer-shell-0' using strategy 'mode_first'. searched paths:
  /usr/local/lib64/libgtk4-layer-shell-0.so
  /usr/local/lib/libgtk4-layer-shell-0.so
  /lib64/libgtk4-layer-shell-0.so
  /lib/libgtk4-layer-shell-0.so
  /usr/lib64/libgtk4-layer-shell-0.so
  /usr/lib/libgtk4-layer-shell-0.so
  /usr/local/lib64/libgtk4-layer-shell-0.a
  /usr/local/lib/libgtk4-layer-shell-0.a
  /lib64/libgtk4-layer-shell-0.a
  /lib/libgtk4-layer-shell-0.a
  /usr/lib64/libgtk4-layer-shell-0.a
  /usr/lib/libgtk4-layer-shell-0.a

[vercel]

@SaudAlshareef is attempting to deploy a commit to the Ghostty Team on Vercel.

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