refactor: Replace fwd-decl with proper include

This is fine, because the span.h include is lightweight and a proper
include will be needed anyway when switching to std::span.

src/util/hasher.h

@@ -8,13 +8,12 @@
 #include <crypto/common.h>
 #include <crypto/siphash.h>
 #include <primitives/transaction.h>
+#include <span.h>
 #include <uint256.h>
 
 #include <cstdint>
 #include <cstring>
 
-template <typename C> class Span;
-
 class SaltedTxidHasher
 {
 private: