missing declaration when FOONATHAN_MEMORY_BUILD_TOOLS=OFF

[briansoe66]

Setting cmake option FOONATHAN_MEMORY_BUILD_TOOLS=OFF results in an empty header for container_node_sizes_impl.hpp
This results in a build error when including container.hpp, because shared_ptr_stateful_node_size has no definition.

My motivation for setting FOONATHAN_MEMORY_BUILD_TOOLS=OFF is to build for QNX, which does not contain experimental/memory_resource.h