Break circular dependency: chain -> pow -> chain

chain.h does not actually depend on the methods defined in pow.h, just its
include of consensus/params.h, which is standalone and can be included instead.

Confirmed by inspection and successful build.

src/test/test_bitcoin.cpp

@@ -11,6 +11,7 @@
 #include <validation.h>
 #include <miner.h>
 #include <net_processing.h>
+#include <pow.h>
 #include <ui_interface.h>
 #include <streams.h>
 #include <rpc/server.h>