selftests/futex: Add futex_numa to .gitignore

futex_numa was never added to the .gitignore file.
Add it.

Fixes: 9140f57c ("futex,selftests: Add another FUTEX2_NUMA selftest")
Signed-off-by: Terry Tritton <terry.tritton@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/all/20250704103749.10341-1-terry.tritton@linaro.org