unix: add uv_get_constrained_memory() cygwin stub

PR-URL: https://github.com/libuv/libuv/pull/2361
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

src/unix/cygwin.c

@@ -52,3 +52,7 @@ void uv_free_cpu_info(uv_cpu_info_t* cpu_infos, int count) {
   (void)cpu_infos;
   (void)count;
 }
+
+uint64_t uv_get_constrained_memory(void) {
+  return 0;  /* Memory constraints are unknown. */
+}