sched_ext: Make cast_mask() inline

cast_mask() doesn't do any actual work and is defined in a header file.
Force it to be inline. When it is not inlined and the function is not used,
it can cause verificaiton failures like the following:

  # tools/testing/selftests/sched_ext/runner -t minimal
  ===== START =====
  TEST: minimal
  DESCRIPTION: Verify we can load a fully minimal scheduler
  OUTPUT:
  libbpf: prog 'cast_mask': missing BPF prog type, check ELF section name '.text'
  libbpf: prog 'cast_mask': failed to load: -22
  libbpf: failed to load object 'minimal'
  libbpf: failed to load BPF skeleton 'minimal': -22
  ERR: minimal.c:20
  Failed to open and load skel
  not ok 1 minimal #
  =====  END  =====

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: a748db0c ("tools/sched_ext: Receive misc updates from SCX repo")