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/blockencodings_tests.cpp

@@ -5,6 +5,7 @@
 #include <blockencodings.h>
 #include <consensus/merkle.h>
 #include <chainparams.h>
+#include <pow.h>
 #include <random.h>
 
 #include <test/test_bitcoin.h>