Move compat/assumptions.h include to one place that actually needs it

Also add the <IWYU pragma: keep> to avoid removing it by accident.

src/common/system.h

@@ -10,7 +10,6 @@
 #include <config/bitcoin-config.h>
 #endif
 
-#include <compat/assumptions.h>
 #include <compat/compat.h>
 
 #include <set>

src/serialize.h

@@ -7,7 +7,10 @@
 #define BITCOIN_SERIALIZE_H
 
 #include <attributes.h>
+#include <compat/assumptions.h> // IWYU pragma: keep
 #include <compat/endian.h>
+#include <prevector.h>
+#include <span.h>
 
 #include <algorithm>
 #include <cstdint>
@@ -18,13 +21,9 @@
 #include <memory>
 #include <set>
 #include <string>
-#include <string.h>
 #include <utility>
 #include <vector>
 
-#include <prevector.h>
-#include <span.h>
-
 /**
  * The maximum size of a serialized object in bytes or number of elements
  * (for eg vectors) when the size is encoded as CompactSize.