Fix dr_insert_clean_call callee arg in tutorial (#6111)

Fixes the callee arg in the dr_insert_clean_call call in the dynamic
basic block size tutorial. The callee pointer should be cast to `void*`
for it to compile successfully.

Related to: https://groups.google.com/g/dynamorio-users/c/BWVdySjLUn8

api/docs/tutorial.dox

@@ -300,8 +300,8 @@ event_basic_block(void *drcontext, void *tag, instrlist_t *bb,
     dr_mutex_unlock(as_built_lock);
 
 +     /* insert clean call */
-+     dr_insert_clean_call(drcontext, bb, instrlist_first(bb), clean_call, false, 1,
-+                          OPND_CREATE_INT32(num_instructions));
++     dr_insert_clean_call(drcontext, bb, instrlist_first(bb), (void *)clean_call,
++                          false, 1, OPND_CREATE_INT32(num_instructions));
 +
     return DR_EMIT_DEFAULT;
 }