debug: move internal functions declarations to an internal header file

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/suites/test_suite_debug.function b/tests/suites/test_suite_debug.function
index b961040..eeefc95 100644
--- a/tests/suites/test_suite_debug.function
+++ b/tests/suites/test_suite_debug.function
@@ -1,5 +1,5 @@
 /* BEGIN_HEADER */
-#include "mbedtls/debug.h"
+#include "debug_internal.h"
 #include "string.h"
 #include "mbedtls/pk.h"