sched_ext: Add __weak to fix the build errors

sched_ext: Add __weak to fix the build errors

commit 5cbb302880f5 ("sched_ext: Rename
scx_bpf_dispatch[_vtime]_from_dsq*() -> scx_bpf_dsq_move[_vtime]*()")
introduced several new functions which caused compilation errors when
compiled with clang.

Let's fix this by adding __weak markers.

Signed-off-by: Honglei Wang <jameshongleiwang@126.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 5cbb302880f5 ("sched_ext: Rename scx_bpf_dispatch[_vtime]_from_dsq*() -> scx_bpf_dsq_move[_vtime]*()")
Acked-by: Andrii Nakryiko <andrii@kernel.org>