powerpc/rtas: Replace one-element array with flexible array member

powerpc/rtas: Replace one-element array with flexible array member

Replace the deprecated one-element array with a modern flexible array
member in the struct rtas_error_log and add the __counted_by_be()
compiler attribute to improve access bounds-checking via
CONFIG_UBSAN_BOUNDS and CONFIG_FORTIFY_SOURCE.

Link: https://github.com/KSPP/linux/issues/79
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250813103101.163698-2-thorsten.blum@linux.dev