watch: Add missing functional include

std::function is not found with a GCC 7.2 runtime.

Tested: Built repository with GCC 7.2
Change-Id: Ie139a8897aea4036c1159bebb03c3b446fb23687
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/watch.hpp b/watch.hpp
index 99854b7..f7eb695 100644
--- a/watch.hpp
+++ b/watch.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <functional>
 #include <systemd/sd-event.h>
 
 namespace phosphor