kmemleak: Select DEBUG_FS unconditionally in DEBUG_KMEMLEAK

In the past DEBUG_FS used to depend on SYSFS and DEBUG_KMEMLEAK selected
it conditionally. This is no longer the case, so always select DEBUG_FS
via DEBUG_KMEMLEAK.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>