Create threading_alt.h header for testing

Follow-up to "Create xxx_alt.h headers for testing". The inclusion of
threading_alt.h in include/mbedtls/threading.h does not follow the
same pattern as the others so it was missed by the script.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/include/alt-dummy/threading_alt.h

@@ -0,0 +1,27 @@
+/* threading_alt.h with dummy types for MBEDTLS_THREADING_ALT */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, 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.
+ */
+
+#ifndef THREADING_ALT_H
+#define THREADING_ALT_H
+
+typedef struct mbedtls_threading_mutex_t
+{
+    int dummy;
+} mbedtls_threading_mutex_t;
+
+#endif /* threading_alt.h */