lib/nfs/Manager: add Compare(ManagedConnection, ManagedConnection)

Required for Boost 1.61, which uses that overload in a BOOST_ASSERT().

src/lib/nfs/Manager.hxx

@@ -64,6 +64,10 @@ class NfsManager final : IdleMonitor {
 		gcc_pure
 		bool operator()(const ManagedConnection &a,
 				const LookupKey b) const;
+
+		gcc_pure
+		bool operator()(const ManagedConnection &a,
+				const ManagedConnection &b) const;
 	};
 
 	/**