[LICENSE] Add SPDX Identifier
diff --git a/verilog/dv/caravel/stub.c b/verilog/dv/caravel/stub.c
index 28090ca..c89330c 100644
--- a/verilog/dv/caravel/stub.c
+++ b/verilog/dv/caravel/stub.c
@@ -12,6 +12,7 @@
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
+ * SPDX-License-Identifier: Apache-2.0
  */
 
 void putchar(char c)