tools/vm: add .gitignore to ignore built binaries

There is no .gitignore in tools/vm, so 'git status' always show built
binaries.  To ignore this, add .gitignore.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>