fix: simdpp/types/empty_expr.h: c++17 compilation fix

[michalfapso]

Compiling with gcc -std=c++17 gave an error message.

[p12tic]

Thanks, and sorry for the delay in reviewing. I've merged #141 which fixes this same issue.

[peabody-korg]

would it be possible to merge this into the dev branch and/or synchronize the dev and master branches?

[peabody-korg]

also I get the same error in simdpp/types/generic.h xcode 13.2 at line:

simdpp/types/generic.h:244:52: No viable conversion from returned value of type 'mask_int32<4U>' to function return type 'uint32<4U>'

SIMDPP_INL operator uint32<N>() const { return eval(); }

The above pull request does not fix that. casting the result with uint32<N>(eval()) appears to resolve the issue.

[peabody-korg]

there's at least one additional error in empty_expr.h as well. class mask_int64<N, expr_empty> has a similar issue, and the cast solution also appears to resolve it.