kernel: sched: fix build breakage when PELT enabled

Because mark_reserved use for WALT and it's called by load_balance,
it leads to build breakage when WALT disabled. Executing the function
only if CONFIG_SCHED_WALT enabled.

Bug: 144142283
Test: Build and boot to home
Change-Id: I5cc3e3ece6a28c6cdabbe6964f6a6032ff2ea809
Signed-off-by: Kyle Lin <kylelin@google.com>
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>