selftests: add .gitignore for vm

Signed-off-by: NRamkumar Ramachandra <artagnon@gmail.com>
Cc: Dave Young <dyoung@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/.gitignore

+hugepage-mmap
+hugepage-shm
+map_hugetlb
+thuge-gen