IP4.h: add conditional implementation of operator <=>

If the compiler has it, we can create it very simply, so just do it
with an appropriate #ifdef.

esp32/max80/IP4.h

@@ -80,6 +80,11 @@ public:
     constexpr bool operator > (const IP4 &b) const {
 	return netswap(l) > netswap(b.l);
     }
+#ifdef __cpp_impl_three_way_comparison
+    constexpr auto operator <=> (const IP4 &b) const {
+	return netswap(l) <=> netswap(b.l);
+    }
+#endif
 
     constexpr IP4 operator & (const IP4 &b) const { return IP4(l & b.l); }
     constexpr IP4 operator | (const IP4 &b) const { return IP4(l | b.l); }