selftests: harness: Mark functions without prototypes static

With -Wmissing-prototypes the compiler will warn about non-static
functions which don't have a prototype defined.
As they are not used from a different compilation unit they don't need to
be defined globally.

Avoid the issue by marking the functions static.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250505-nolibc-kselftest-harness-v4-4-ee4dd5257135@linutronix.de


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>