Call try_flags() even when there are no flags to try

Avoids:

fellow_io_uring.c:234:1: error: ‘try_flag’ defined but not used [-Werror=unused-function]
  234 | try_flag(unsigned flag)
      | ^~~~~~~~

src/fellow_io_uring.c

@@ -256,6 +256,7 @@ try_flags(void)
 
 	if (flags_checked != 0)
 		return;
+	AZ(try_flag(0));
 #ifdef IORING_SETUP_COOP_TASKRUN
 	flags_supported |= try_flag(IORING_SETUP_COOP_TASKRUN);
 #endif