selftests: tty: add tty_tiocsti_test to .gitignore

Building the tty selftests generates the tty_tiocsti_test binary, which
appears as untracked file in git. As mentioned in the kselftest
documentation, all the generated objects must be placed inside
.gitignore. This prevents the generated objects from accidentally
getting staged and keeps the working tree clean.

Add the tty_tiocsti_test binary to .gitignore to avoid accidentally
staging the build artifact and maintain a clean working tree.

Link: https://docs.kernel.org/dev-tools/kselftest.html#contributing-new-tests-details

Fixes: 7553f5173ec3 ("selftests/tty: add TIOCSTI test suite")
Suggested-by: Greg KH <gregkh@linuxfoundation.org>
Suggested-by: David Hunter <david.hunter.linux@gmail.com>
Signed-off-by: Gopi Krishna Menon <krishnagopi487@gmail.com>
Link: https://patch.msgid.link/20251026100104.3354-1-krishnagopi487@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>