include: Drop update.h

update.h is a convenience header to include headers commonly needed in
update code.  The price for the convenience is superfluous recompiles.
Include necessary headers directly, and drop update.h

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

src/lib/update/prepare.c

@@ -37,11 +37,15 @@
 
 #include "budg.h"
 #include "chance.h"
+#include "file.h"
 #include "item.h"
 #include "land.h"
+#include "nat.h"
+#include "optlist.h"
 #include "player.h"
+#include "prototypes.h"
+#include "sect.h"
 #include "ship.h"
-#include "update.h"
 
 void
 prepare_sects(int etu)