Add gitignore file for samples/fanotify/ subdirectory

Commit 54510930 ("samples: Add fs error monitoring example") added a
new sample program, but didn't teach git to ignore the new generated
files, causing unnecessary noise from 'git status' after a full build.

Add the 'fs-monitor' sample executable to the .gitignore for this
subdirectory to silence it all again.

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