sched: create unique function name

In file include/kernel/thread.h in "struct _thread_base" is a member
called "_wait_q_t *pended_on"
At the same time in file kernel/sched.c is function called
"static _wait_q_t *pended_on()"

Coding scanning tool assigns violation (MISRA R5.9) that static
object reused, because thread.h is included in struct.c file.

I think we can rename function to avoid misreading in the future.

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>