perf kmem: Fix unused argument build warning

Fix:

 builtin-kmem.c: In function 'parse_caller_opt':
 builtin-kmem.c:690: error: unused parameter 'arg'
 builtin-kmem.c: In function 'parse_alloc_opt':
 builtin-kmem.c:697: error: unused parameter 'arg'

Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
LKML-Reference: <4B20A195.8030106@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>