sched_ext: Add __weak markers to BPF helper function decalarations

Fix build errors by adding __weak markers to BPF helper function
declarations in header files. This resolves static assertion failures
in scx_qmap.bpf.c and scx_flatcg.bpf.c where functions like
scx_bpf_dispatch_from_dsq_set_slice, scx_bpf_dispatch_from_dsq_set_vtime,
and scx_bpf_task_cgroup were missing the __weak attribute.

[1] https://lore.kernel.org/all/ZvvfUqRNM4-jYQzH@linux.ibm.com



Signed-off-by: Vishal Chourasia <vishalc@linux.ibm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>