[skein] Fix compile issue with unknown symbol SKEIN_ASM_UNROLL1024

Weirdly, I needed to sprinkle more parens here to get gcc-as in 6.4
to correctly generate things.

Without them, I'd get an unknown variable reference to SKEIN_ASM_UNROLL1024.

This at least links now, but I haven't run any test cases against it.
It may be worthwhile doing it in case gcc-as demands we liberally sprinkle
more brackets around variables in .if statements.

Thanks to ed for the suggestion of just sprinkling more brackets to
see if that helped.

Reviewed by: emaste