openbsd: fix siginfo_t struct definition
openbsd: fix siginfo_t struct definition

`_proc` struct part contains an union for kill/cld parts.

see [siginfo_t](https://github.com/openbsd/src/blob/77c6c13150aaa9f0a29fe29b233c4436d1da01c0/sys/sys/siginfo.h#L132)

zig-lang/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig