usr.bin/diff/tests/diff_test: add SPDX ID

The SPDX ID makes the script's licensing more explicit.

MFC after:	1 week
Reviewed by:	bapt
Differential Revision:	https://reviews.freebsd.org/D60908

usr.bin/diff/tests/diff_test.sh

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: BSD-2-Clause
+
 atf_test_case simple
 atf_test_case unified
 atf_test_case header