Include missing <ostream>

Cheney-W · web-flow · commit 023c93e4e504 · 2020-02-11T10:07:45.000-08:00
diff --git a/core/src/histogram.cc b/core/src/histogram.cc
@@ -4,6 +4,7 @@
 #include <cassert>
 #include <iterator>
 #include <numeric>
+#include <ostream>
 
 namespace prometheus {
 
