internal/syscall/unix: add build constraint to nonblocking_js.go
The intention was for this file to be constrained to both js and wasm,
but the build constraint was missing, causing it to be constrained only
to js because of the _js suffix in the filename.
Add a js,wasm build constraint. The js part is redundant, but specified
anyway to make it more visible and consistent with other similar files.
This issue was spotted while working on GopherJS, because it was causing
a conflict there (both nonblocking.go and nonblocking_js.go files were
being matched).