Support Linux kernel earilier version than v4.11

Support Linux kernel earilier version than v4.11

From Linux kernel 4.11, sched header is splited and cputime_t is
fixed as u64. Before the kernel version, do not use splitter
headers and use cputime_t for task_cputime_adjusted().

Change-Id: Iac5d4e1d89366c54a59cc3092afd56f2df01918b
Ref: v4.11 commit 1050b27c52f6 ("sched/headers: Move cputime functionality from <linux/sched.h> and <linux/cputime.h> into <linux/sched/cputime.h>")
Ref: v4.11 commit c3edc4010e9d ("sched/headers: Move task_struct::signal and task_struct::sighand types and accessors into <linux/sched/signal.h>")
Ref: v4.11 commit 5613fda9a503 ("sched/cputime: Convert task/group cputime to nsecs")
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>