Add missing includes which broke macOS clang build.

Should help with #416, but I have a feeling this may not be the whole
story.  One step at a time.

NEWS

@@ -1,3 +1,5 @@
+7.4.1
+ - Missing includes; broke macOS clang build.  (#416)
 7.4.0
  - Work around Visual Studio 2017 bug with `[[deprecated]]`.  (#405, #406)
  - Work around eternal Windows bug with `max` macro yet again.  (#101)

include/pqxx/connection.hxx

@@ -33,6 +33,7 @@
 #include "pqxx/errorhandler.hxx"
 #include "pqxx/except.hxx"
 #include "pqxx/prepared_statement.hxx"
+#include "pqxx/separated_list.hxx"
 #include "pqxx/strconv.hxx"
 #include "pqxx/util.hxx"
 #include "pqxx/zview.hxx"

include/pqxx/stream_to.hxx

@@ -16,6 +16,7 @@
 #include "pqxx/compiler-public.hxx"
 #include "pqxx/internal/compiler-internal-pre.hxx"
 
+#include "pqxx/separated_list.hxx"
 #include "pqxx/transaction_base.hxx"