sched: qhmp: Fix compilation for IRQ_TIME_ACCOUNTING

When IRQ_TIME_ACCOUNTING feature is enabled, compiler gives an
undefined reference error for sched_account_irqstart function.
Since cycle counter based accounting is not needed for any targets
that are using QHMP scheduler, provide a nop definition for this
function.

Change-Id: Id34e26c4e8d38db953e5d8e1f527e20a1772fe3a
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>