timer: Add missing functional include

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

Change-Id: Ic7eaf4012a2992ee4a58f2dda827f0625a9ae96f
Tested: Built repository with GCC 7.2
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/timer.hpp b/timer.hpp
index e6b0693..058740e 100644
--- a/timer.hpp
+++ b/timer.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <functional>
 #include <memory>
 #include <chrono>
 #include <systemd/sd-event.h>