Cleanup: Use const for internal file data of asset-handle

Note that the current asset-handle design is temporary, see
35affaa971. I still prefer this to be const, as code outside the
asset-list/file-list code should never mess with the file data of an
asset.

source/blender/makesdna/DNA_asset_types.h

@@ -116,7 +116,7 @@ typedef struct AssetLibraryReference {
 #
 #
 typedef struct AssetHandle {
-  struct FileDirEntry *file_data;
+  const struct FileDirEntry *file_data;
 } AssetHandle;
 
 #ifdef __cplusplus

source/blender/makesrna/intern/rna_asset.c

@@ -129,7 +129,9 @@ static void rna_AssetMetaData_active_tag_range(
 static PointerRNA rna_AssetHandle_file_data_get(PointerRNA *ptr)
 {
   AssetHandle *asset_handle = ptr->data;
-  return rna_pointer_inherit_refine(ptr, &RNA_FileSelectEntry, asset_handle->file_data);
+  /* Have to cast away const, but the file entry API doesn't allow modifications anyway. */
+  return rna_pointer_inherit_refine(
+      ptr, &RNA_FileSelectEntry, (FileDirEntry *)asset_handle->file_data);
 }
 
 static void rna_AssetHandle_file_data_set(PointerRNA *ptr,

source/blender/makesrna/intern/rna_context.c

@@ -146,7 +146,9 @@ static PointerRNA rna_Context_asset_file_handle_get(PointerRNA *ptr)
   }
 
   PointerRNA newptr;
-  RNA_pointer_create(NULL, &RNA_FileSelectEntry, asset_handle.file_data, &newptr);
+  /* Have to cast away const, but the file entry API doesn't allow modifications anyway. */
+  RNA_pointer_create(
+      NULL, &RNA_FileSelectEntry, (struct FileDirEntry *)asset_handle.file_data, &newptr);
   return newptr;
 }