alpha: Replace one-element array with flexible-array member

One-element and zero-length arrays are deprecated. So, replace
one-element array in struct osf_dirent with flexible-array
member.

This results in no differences in binary output.

Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZMpZZBShlLqyD3ax@work


Signed-off-by: Kees Cook <keescook@chromium.org>