binderfs: add gitignore for generated sample program

Let's keep "git status" happy and quiet.

Fixes: 9762dc14 ("samples: add binderfs sample program
Fixes: fca5e949 ("samples: binderfs: really compile this sample and fix build issues")
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

samples/binderfs/.gitignore

+binderfs_example