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/test.c

@@ -11,6 +11,8 @@
 #include <linux/kernel.h>
 #include <linux/sched/debug.h>
 
+#include "string-stream.h"
+
 static void kunit_set_failure(struct kunit *test)
 {
 	WRITE_ONCE(test->success, false);