kunit: move string-stream.h to lib/kunit

string-stream interfaces are not intended for external use;
move them from include/kunit to lib/kunit accordingly.

Co-developed-by: Knut Omang <knut.omang@oracle.com>
Signed-off-by: Knut Omang <knut.omang@oracle.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

lib/kunit/string-stream.c

@@ -6,11 +6,12 @@
  * Author: Brendan Higgins <brendanhiggins@google.com>
  */
 
-#include <kunit/string-stream.h>
 #include <kunit/test.h>
 #include <linux/list.h>
 #include <linux/slab.h>
 
+#include "string-stream.h"
+
 struct string_stream_fragment_alloc_context {
 	struct kunit *test;
 	int len;