nstree: decouple from ns_common header
Foward declare struct ns_common and remove the include of ns_common.h.
We want ns_common.h to possibly include nstree structures but not the
other way around.
Link: https://patch.msgid.link/20251110-work-namespace-nstree-fixes-v1-2-e8a9264e0fb9@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>