Create threading_internal.h

This is meant to hold threading-related definitions that are not public, but
are used in the test framework.

To be populated later.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/threading.c b/library/threading.c
index fde7cea..ff8183e 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -17,7 +17,7 @@
 
 #if defined(MBEDTLS_THREADING_C)
 
-#include "mbedtls/threading.h"
+#include "threading_internal.h"
 
 #if defined(MBEDTLS_HAVE_TIME_DATE) && !defined(MBEDTLS_PLATFORM_GMTIME_R_ALT)