util/winutil: make it actually compile

Helps to use the right GOOS after refactoring, sigh.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/winutil/winutil.go

@@ -7,6 +7,10 @@
 // Package winuntil contains misc Windows/win32 helper functions.
 package winutil
 
+import (
+	"golang.org/x/sys/windows"
+)
+
 // GetDesktopPID searches the PID of the process that's running the
 // currently active desktop and whether it was found.
 // Usually the PID will be for explorer.exe.
@@ -15,7 +19,6 @@ func GetDesktopPID() (pid uint32, ok bool) {
 	if hwnd == 0 {
 		return 0, false
 	}
-	var pid uint32
 	windows.GetWindowThreadProcessId(hwnd, &pid)
 	return pid, pid != 0
 }