Refactor: fix wrong struct typedef name in DNA_action_types

Fix `typedef struct ActionChannelBag { ... } ChannelBag;` so that the
'typedef name' is also `ActionChannelBag`.

Note that this does _not_ change the struct name in DNA. It's purely for
the naming in our code.

No functional changes.

source/blender/makesdna/DNA_action_types.h

@@ -1276,7 +1276,7 @@ typedef struct ActionChannelBag {
   blender::animrig::ChannelBag &wrap();
   const blender::animrig::ChannelBag &wrap() const;
 #endif
-} ChannelBag;
+} ActionChannelBag;
 
 #ifdef __cplusplus
 /* Some static assertions that things that should have the same type actually do. */

source/blender/makesrna/intern/rna_action.cc

@@ -484,7 +484,7 @@ static ActionChannelBag *rna_ChannelBags_new(KeyframeActionStrip *dna_strip,
     return nullptr;
   }
 
-  ChannelBag &channelbag = key_strip.channelbag_for_slot_add(slot);
+  animrig::ChannelBag &channelbag = key_strip.channelbag_for_slot_add(slot);
 
   WM_event_add_notifier(C, NC_ANIMATION | ND_ANIMCHAN, nullptr);
   /* No need to tag the depsgraph, as there is no new animation yet. */