poly1305: fix build

Updates golang/go#17422

Change-Id: Ie5f16e24f87b3d800f1182b5b09d6cf638135e33
Reviewed-on: https://go-review.googlesource.com/30970
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

poly1305/sum_arm.s

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-#include "textflags.h"
+#include "textflag.h"
 
 // This code was translated into a form compatible with 5a from the public
 // domain source by Andrew Moon: github.com/floodyberry/poly1305-opt/blob/master/app/extensions/poly1305.