MAINTAINERS/MEMORY MANAGEMENT: add tools/vm/ as managed files

'tools/vm/' directory should be a part of memory management subsystem, but
MAINTAINERS file doesn't mark the directory so. Add one more 'F:' entry
for the directory to 'MEMORY MANAGEMENT' section.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: SeongJae Park <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

MAINTAINERS

···
 F:	include/linux/pagewalk.h
 F:	mm/
 F:	tools/testing/selftests/vm/
+F:	tools/vm/
 
 VMALLOC
 M:	Andrew Morton <[e-mail]>