Move "Windows-only Warning" out of build.zig

[FObersteiner]

I noted that the warning

skipping example 'list_port_info' - only supported on Windows

from 'list_port_info' being Windows-only pops up whenever serial is called as a dependency in a build. I've put that warning in there, so I feel responsible for cleaning up ;-)

I'd move the warning to the example itself, and make it an error that clarifies what's wrong. The list_port_info example would now also be built on Linux for example, but exit with code 1 and print

error(serial_ex_portinfo): 'list_port_info' example is only supported on Windows

[FObersteiner]

already completed