unix: fix unused-function warning on BSD

This commit fixes a warning about uv__fs_preadv() being
unused on BSD.

PR-URL: https://github.com/libuv/libuv/pull/2367
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/fs.c

@@ -292,6 +292,7 @@ static ssize_t uv__fs_open(uv_fs_t* req) {
 }
 
 
+#if !HAVE_PREADV
 static ssize_t uv__fs_preadv(uv_file fd,
                              uv_buf_t* bufs,
                              unsigned int nbufs,
@@ -338,6 +339,7 @@ static ssize_t uv__fs_preadv(uv_file fd,
 
   return result;
 }
+#endif
 
 
 static ssize_t uv__fs_read(uv_fs_t* req) {