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/[e-mail]
Fixes: 268af17ada5855 ("selftests: proc: test subset=pid")
Signed-off-by: David Matlack <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Cc: Alexey Gladkov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/proc/.gitignore

···
 /proc-self-map-files-002
 /proc-self-syscall
 /proc-self-wchan
+/proc-subset-pid
 /proc-uptime-001
 /proc-uptime-002
 /read