utils: Add missing functional include

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

Tested: Built repository with GCC 7.2
Change-Id: I90d2585bb49973744bc62c68a5b10362478852c7
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/host-ipmid/ipmid-host-cmd-utils.hpp b/host-ipmid/ipmid-host-cmd-utils.hpp
index d95482b..293cc0c 100644
--- a/host-ipmid/ipmid-host-cmd-utils.hpp
+++ b/host-ipmid/ipmid-host-cmd-utils.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <functional>
 #include <unistd.h>
 #include <tuple>