[v3 linux] add non-functional activeInstance implementation

v3/plugins/single_instance/plugin_linux.go

@@ -0,0 +1,17 @@
+//go:build linux
+
+package single_instance
+
+/*
+#cgo CFLAGS:
+#cgo LDFLAGS:
+
+*/
+import "C"
+import "fmt"
+
+func (p *Plugin) activeInstance(pid int) error {
+	//	C.activateApplicationWithProcessID(C.int(pid))
+	fmt.Println("[linux] activateInstance - not implemented: ", pid)
+	return nil
+}