net/SocketAddress: add #ifdefs for std::span

src/net/SocketAddress.cxx

@@ -131,6 +131,8 @@ SocketAddress::GetPort() const noexcept
 	}
 }
 
+#ifdef __cpp_lib_span
+
 static std::span<const std::byte>
 GetSteadyPart(const struct sockaddr_in &address) noexcept
 {
@@ -149,7 +151,11 @@ GetSteadyPart(const struct sockaddr_in6 &address) noexcept
 	};
 }
 
-#endif
+#endif // __cpp_lib_span
+
+#endif // HAVE_TCP
+
+#ifdef __cpp_lib_span
 
 std::span<const std::byte>
 SocketAddress::GetSteadyPart() const noexcept
@@ -175,3 +181,5 @@ SocketAddress::GetSteadyPart() const noexcept
 		return {};
 	}
 }
+
+#endif