Remove event_subscriber declaration from event_bus.h

[kevingranade]

Summary

SUMMARY: None

Purpose of change

Continuing to try to improve build times.

Describe the solution

Isolate the event_subscriber declaration from the other event declarations so forward includes are sufficient for declaring new subscribers.

Describe alternatives you've considered

pimpl-ing the magic class, pimpling the character class.

Testing

Needs to build.
Expecting CBA to show a mild improvement in character.h overhead, in the neighbourhood of 40s

[anothersimulacrum]

 94992 ms: src/character.h (included 142 times, avg 668 ms), included via:
  talker_avatar.o avatar.h  (1007 ms)
  timed_event.o avatar.h  (992 ms)
  character_crafting.o  (987 ms)
  action.o avatar.h  (987 ms)
  fungal_effects.o avatar.h  (982 ms)
  melee.o avatar.h  (979 ms)
  ...

56988 ms: ../src/character.h (included 62 times, avg 919 ms), included via:
  reading_test.o avatar.h  (1133 ms)
  itemname_test.o  (1097 ms)
  comestible_test.o  (1078 ms)
  item_location_test.o  (1069 ms)
  player_test.o  (1056 ms)
  active_item_test.o avatar.h  (1048 ms)
  ...

Looks like #42680 has a greater impact, but I'm not sure how much of that is actually relevant.

 90783 ms: src/character.h (included 142 times, avg 639 ms), included via:
  talker_npc.o avatar.h  (945 ms)
  monexamine.o avatar.h  (943 ms)
  talker_avatar.o avatar.h  (935 ms)
  melee.o avatar.h  (929 ms)
  map_field.o avatar.h  (927 ms)
  activity_item_handling.o avatar.h  (922 ms)
  ...
53129 ms: ../src/character.h (included 62 times, avg 856 ms), included via:
  item_location_test.o  (1008 ms)
  item_tname_test.o  (1004 ms)
  itemname_test.o  (1002 ms)
  comestible_test.o  (994 ms)
  vehicle_split_test.o  (986 ms)
  iuse_test.o avatar.h  (966 ms)
  ...