pkg/term/windows: add missing build-tag

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/term/windows/windows.go

@@ -1,3 +1,4 @@
+// +build windows
 // These files implement ANSI-aware input and output streams for use by the Docker Windows client.
 // When asked for the set of standard streams (e.g., stdin, stdout, stderr), the code will create
 // and return pseudo-streams that convert ANSI sequences to / from Windows Console API calls.