xorg-server-xwayland: enable libdecor for rootful Xwayland

[Calandracas606]

Testing the changes

• I tested the changes in this PR: briefly

Local build testing

• I built this PR locally for my native architecture, x86_64-glibc
• I built this PR locally for these architectures (if supported. mark crossbuilds):
  • x86_64-musl
  • i686
  • aarch64 (cross)
  • aarch64-musl (cross)
  • armv7l (cross)
  • armv7l-musl (cross)
  • armv6l (cross)
  • armv6l-musl (cross)