runtime: add missing copyright header to signal_windows_test.go

runtime: add missing copyright header to signal_windows_test.go

The file was added in CL 181839 which was submitted in 2019.

Also delete the superfluous //go:build windows build constraint which is
already part of the file name.

Change-Id: Iecdf3d75848a7b8e2a76cd18c9fff4fb90acdac2
Reviewed-on: https://go-review.googlesource.com/c/go/+/427854
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>