build: make code compilable for iOS on Xcode

Fixes: https://github.com/libuv/libuv/issues/2638
PR-URL: https://github.com/libuv/libuv/pull/2639
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

src/unix/core.c

@@ -533,7 +533,7 @@ int uv__close_nocancel(int fd) {
 #if defined(__APPLE__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdollar-in-identifier-extension"
-#if defined(__LP64__)
+#if defined(__LP64__) || defined(TARGET_OS_IPHONE)
   extern int close$NOCANCEL(int);
   return close$NOCANCEL(fd);
 #else