From flatbuffers upstream: Fix compiler error

Original author of change: avaliente-bc
Backport/update from upstream flatbuffers repository.
Change taken from google/flatbuffers#7227

Author: ltoenning

include/behaviortree_cpp_v3/flatbuffers/stl_emulation.h

@@ -625,7 +625,7 @@ class span FLATBUFFERS_FINAL_CLASS {
  private:
   // This is a naive implementation with 'count_' member even if (Extent != dynamic_extent).
   pointer const data_;
-  const size_type count_;
+  size_type count_;
 };
 
  #if !defined(FLATBUFFERS_SPAN_MINIMAL)