sched: Make clangd usable

sched: Make clangd usable

Due to the weird Makefile setup of sched the various files do not
compile as stand alone units. The new generation of editors are trying
to do just this -- mostly to offer fancy things like completions but
also better syntax highlighting and code navigation.

Specifically, I've been playing around with neovim and clangd.

Setting up clangd on the kernel source is a giant pain in the arse
(this really should be improved), but once you do manage, you run into
dumb stuff like the above.

Fix up the scheduler files to at least pretend to work.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20250523164348.GN39944@noisy.programming.kicks-ass.net