lib: Add SPDX identifiers to files missing them

This adds SPDX identifiers to the remaining source files in the
lib directory that don't already have them.

A note on gcov-iov.h - As machine generated content, this file is
believed to be uncopyrightable, and therefore in the public domain, so
gets the CC-PDDC license even though there is code in the file.

Signed-off-by: Martin Roth <martin.roth@amd.corp-partner.google.com>
Change-Id: Ifcb584d78a55e56c1b5c02d424a7e950a7f115dc
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66502
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>

src/lib/string.c

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
 #include <assert.h>
 #include <ctype.h>
 #include <string.h>