fix crash in single-threaded builds

lib/std/event/loop.zig

@@ -616,6 +616,7 @@ pub const Loop = struct {
 
         self.workerRun();
 
+        if (!builtin.single_threaded) {
             switch (builtin.os.tag) {
                 .linux,
                 .macosx,
@@ -625,6 +626,7 @@ pub const Loop = struct {
                 => self.fs_thread.wait(),
                 else => {},
             }
+        }
 
         for (self.extra_threads) |extra_thread| {
             extra_thread.wait();