proc: add .gitignore for proc-subset-pid selftest

This new selftest needs an entry in the .gitignore file otherwise git
will try to track the binary.

Link: https://lkml.kernel.org/r/20210601164305.11776-1-dmatlack@google.com
Fixes: 268af17a

 ("selftests: proc: test subset=pid")
Signed-off-by: David Matlack <dmatlack@google.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Alexey Gladkov <gladkov.alexey@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>