Fix GCC 7 compile error

jupp0r · jupp0r · commit 05135163b74c · 2018-04-21T13:22:39.000-07:00
This change fixes #107 by adding a missing include causing compile erros on GCC 7. Thanks to Romain Sertelon for reporting this.
diff --git a/include/prometheus/summary.h b/include/prometheus/summary.h
@@ -3,6 +3,7 @@
 #include <array>
 #include <atomic>
 #include <chrono>
+#include <functional>
 #include <list>
 #include <mutex>
 #include <vector>
