Fix build without metadata support

Commit f36352dc4e introduced the parameter
pipe_timeout for the metadata config. If shairport-sync is now configured
without metadata support, the build fails with:

```
shairport.c: In function ‘main’:
shairport.c:766:9: error: ‘shairport_cfg {aka struct <anonymous>}’ has no member named ‘metadata_pipe_timeout’
   config.metadata_pipe_timeout = 5000; //milliseconds

```

Fix this by opting-out the affected code with CONFIG_METADATA.

common.c

@@ -513,6 +513,7 @@ uint64_t get_absolute_time_in_fp() {
   return time_now_fp;
 }
 
+#ifdef CONFIG_METADATA
 ssize_t non_blocking_write(int fd, const void *buf, size_t count) {
 	void *ibuf = (void *)buf;
 	size_t bytes_remaining = count;
@@ -547,6 +548,7 @@ ssize_t non_blocking_write(int fd, const void *buf, size_t count) {
 		return rc;
   //  return write(fd,buf,count);
 }
+#endif
 
 /* from http://coding.debuntu.org/c-implementing-str_replace-replace-all-occurrences-substring#comment-722 */