count: Add missing functional include

std::function is not found with a GCC 7.3 runtime.

Change-Id: I4e5289794d405c42ab6291c211289d1ab175b197
Tested: Built repository with GCC 7.3
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/src/count.hpp b/src/count.hpp
index bde8a91..0da0324 100644
--- a/src/count.hpp
+++ b/src/count.hpp
@@ -4,6 +4,7 @@
 #include "data_types.hpp"
 
 #include <algorithm>
+#include <functional>
 
 namespace phosphor
 {