elf, uapi: Add definition for STN_UNDEF

The definition is used by tools/testing/selftests/vDSO/parse_vdso.c.

To be able to build the vDSO selftests without a libc dependency,
add the definition to the kernels own UAPI headers.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://refspecs.linuxfoundation.org/elf/gabi4+/ch4.symtab.html
Link: https://lore.kernel.org/all/20250226-parse_vdso-nolibc-v2-2-28e14e031ed8@linutronix.de