build: fix go mod tidy removing golang.org/x/mobile dependency

Before this running `go mod tidy` caused the build to break because it
removed the dependency on golang.org/x/mobile and a command line tool
from this package is needed for the build.

This adds an explicit dependency which will mean the tool is always present.

librclone/gomobile/gomobile.go

@@ -6,6 +6,8 @@ import (
 
 	_ "github.com/rclone/rclone/backend/all" // import all backends
 	_ "github.com/rclone/rclone/lib/plugin"  // import plugins
+
+	_ "golang.org/x/mobile/event/key" // make go.mod add this as a dependency
 )
 
 // RcloneInitialize initializes rclone as a library