core: slim down `common.hpp`

Change-Id: I875c35147edd2261fbaa24e809c170d5cd9b94d3
diff --git a/tests/other/cs-benchmark.cpp b/tests/other/cs-benchmark.cpp
index 4446d31..f642d14 100644
--- a/tests/other/cs-benchmark.cpp
+++ b/tests/other/cs-benchmark.cpp
@@ -26,6 +26,7 @@
 #include "benchmark-helpers.hpp"
 #include "table/cs.hpp"
 
+#include <functional>
 #include <iostream>
 
 #ifdef NFD_HAVE_VALGRIND