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 <[e-mail]>
Fixes: a748db0c8c6a ("tools/sched_ext: Receive misc updates from SCX repo")

tools/sched_ext/include/scx/common.bpf.h

···
 /*
  * Access a cpumask in read-only mode (typically to check bits).
  */
-const struct cpumask *cast_mask(struct bpf_cpumask *mask)
+static __always_inline const struct cpumask *cast_mask(struct bpf_cpumask *mask)
 {
 	return (const struct cpumask *)mask;
 }