sched_ext: Fix function pointer type mismatches in BPF selftests

Fix incompatible function pointer type warnings in sched_ext BPF selftests by
explicitly casting the function pointers when initializing struct_ops.
This addresses multiple -Wincompatible-function-pointer-types warnings from the
clang compiler where function signatures didn't match exactly.

The void * cast ensures the compiler accepts the function pointer
assignment despite minor type differences in the parameters.

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