Make rpc_inc() and rpc_dec() private

include/rcp.h

@@ -275,7 +275,8 @@ rcp<T> rcp_dynamic_cast(rcp<U> && other)
 }
 
 #define rcp_managed_root(classname) \
-    public: \
+    private: \
+        mutable std::atomic<int> ref_count{0}; \
         void rcp_inc() const \
         { \
             ref_count.fetch_add(1, std::memory_order_relaxed); \
@@ -291,8 +292,7 @@ rcp<T> rcp_dynamic_cast(rcp<U> && other)
         { \
             return ref_count.load(std::memory_order_relaxed); \
         } \
-    private: \
-        mutable std::atomic<int> ref_count{0}; \
+        template <typename T> friend class rcp; \
     rcp_managed(classname)
 
 #define rcp_managed(classname) \