Adding include to functional

adding include to functional since we use it in this file. Some users
are reporting build issues due to missing this include

Test: m fastboot
Change-Id: I9ba11bdaf7dcedf5f9ebfb6fa50013be50738f48
diff --git a/fastboot/fastboot.h b/fastboot/fastboot.h
index f6ffb64..dc57149 100644
--- a/fastboot/fastboot.h
+++ b/fastboot/fastboot.h
@@ -27,6 +27,7 @@
  */
 #pragma once
 
+#include <functional>
 #include <string>
 #include "fastboot_driver.h"
 #include "fastboot_driver_interface.h"