Fix boost headers included as user instead of system headers

In most of the project, boost headers are included as system headers.
Fix the few inconsistent places where they aren't.

src/miner.h

@@ -11,8 +11,8 @@
 
 #include <stdint.h>
 #include <memory>
-#include "boost/multi_index_container.hpp"
-#include "boost/multi_index/ordered_index.hpp"
+#include <boost/multi_index_container.hpp>
+#include <boost/multi_index/ordered_index.hpp>
 
 class CBlockIndex;
 class CChainParams;