fuzz: add util/mempool/h.cpp

Moving the mempool code (Boost) out of util.h, results in a ~10% speedup
(for me) when compiling the fuzz tests.

src/test/fuzz/integer.cpp

@@ -12,6 +12,7 @@
 #include <key_io.h>
 #include <memusage.h>
 #include <netbase.h>
+#include <policy/policy.h>
 #include <policy/settings.h>
 #include <pow.h>
 #include <protocol.h>