While the function will never returns, gcc will warns.

Add a return statement to make gcc happy.
Before f44f1e7d we never noticed because gcc knows that longjmp does
not return.

arch/um/os-Linux/skas/process.c: In function ‘start_idle_thread’:
arch/um/os-Linux/skas/process.c:613:1: warning: control reaches end of
non-void function [-Wreturn-type]

Fixes: f44f1e7d ("um: Avoid longjmp/setjmp symbol clashes with
libpthread.a")
Signed-off-by: Richard Weinberger <richard@nod.at>
Patch-mainline: linux-kernel @ 03/29/18, 22:45:59

Change-Id: I6d0879ab5dce4a34552da68d9b55f48478b9e497
Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>