selftests/nolibc: ignore -Wmissing-prototypes

selftests/nolibc: ignore -Wmissing-prototypes

To make sure nolibc itself is compatible with -Wmissing-prototypes the
compiler flag should be enabled when building nolibc-test.
However some of its functions are non-static to ease debugging [0],
triggering the compiler warning.

Disable the warning inside nolibc-test while still enabling it for
nolibc itself.

[0] https://lore.kernel.org/lkml/ZMjM0UPRAqoC+goY@1wt.eu/

Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250123-nolibc-prototype-v1-2-e1afc5c1999a@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>