compiler: use -lpthread during building v itself.

vlib/sync/sync_nix.c.v

@@ -3,6 +3,8 @@
 // that can be found in the LICENSE file.
 module sync
 
+#flag -lpthread
+
 // [init_with=new_mutex] // TODO: implement support for this struct attribute, and disallow Mutex{} from outside the sync.new_mutex() function.
 [ref_only]
 pub struct Mutex {