MAINTAINERS: add lib/tests/slub_kunit.c to SLAB ALLOCATOR section

The slub_kunit module has been maintained by SLAB ALLOCATOR folks,
but is missing in the MAINTAINERS file. Add the missing entry.

Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: SeongJae Park <sj@kernel.org>
Link: https://patch.msgid.link/20260406090907.11710-2-harry@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

Author: hygoni

MAINTAINERS

@@ -24363,6 +24363,7 @@ F:	Documentation/admin-guide/mm/slab.rst
 F:	Documentation/mm/slab.rst
 F:	include/linux/mempool.h
 F:	include/linux/slab.h
+F:	lib/tests/slub_kunit.c
 F:	mm/failslab.c
 F:	mm/mempool.c
 F:	mm/slab.h