sched: Fix a compilation issue when WALT is disabled

find_rtg_target() is a WALT specific function. Accessing it outside
WALT code is resulting in a compilation error. Fix this by adding
a stub for find_rtg_target().

Change-Id: Iccc2c2ef1490e66f7c33b8d6d4afda18c398d438
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>