kmemleak: Scan the _ftrace_events section in modules

This section contains pointers to allocated objects and not scanning it
leads to false positives.

Reported-by: Zdenek Kabelac <zdenek.kabelac@gmail.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>