runtime: use new #include "textflag.h"

I did this just to clean things up, but it will be important
when we drop the pkg directory later.

LGTM=bradfitz
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/132600043
diff --git a/src/pkg/runtime/memmove_386.s b/src/pkg/runtime/memmove_386.s
index 51b08d56..4c0c74c 100644
--- a/src/pkg/runtime/memmove_386.s
+++ b/src/pkg/runtime/memmove_386.s
@@ -25,7 +25,7 @@
 
 // +build !plan9
 
-#include "../../cmd/ld/textflag.h"
+#include "textflag.h"
 
 TEXT runtime·memmove(SB), NOSPLIT, $0-12
 	MOVL	to+0(FP), DI