tools/Makefile: do missed s/vm/mm/

tools/Makefile: do missed s/vm/mm/

Commit 799fb82aa132 ("tools/vm: rename tools/vm to tools/mm") missed
renaming 'vm' in 'tools/Makefile' to 'mm'.  As a result, 'make clean'
under 'tools/' directory fails as below:

    $ make -C tools clean
      DESCEND vm
    make[1]: Entering directory '/linux/tools/vm'
    make[1]: *** No rule to make target 'clean'.  Stop.
    make[1]: Leaving directory '/linux/tools/vm'
    make: *** [Makefile:173: vm_clean] Error 2
    make: Leaving directory '/linux/tools'

Do the missed rename.

Link: https://lkml.kernel.org/r/20230415203110.13858-1-sj@kernel.org
Fixes: 799fb82aa132 ("tools/vm: rename tools/vm to tools/mm")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: Ricardo Pardini <ricardo@pardini.net>
Link: https://lore.kernel.org/linux-mm/20230415202454.13558-1-sj@kernel.org/
Tested-by: Ricardo Pardini <ricardo@pardini.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>