Add gitignore file for samples/vfs/ generated files

Commit f1b5618e ("vfs: Add a sample program for the new mount API")
added sample programs that get built during the kernel build, but then
cause 'git status' to worry about whether the resulting binaries should
be managed by git.

Tell git not to worry, and to ignore the sample binaries.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>