debug: move internal functions declarations to an internal header file

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_tls12_client.c b/library/ssl_tls12_client.c
index 0c5af87..c3a8037 100644
--- a/library/ssl_tls12_client.c
+++ b/library/ssl_tls12_client.c
@@ -14,7 +14,7 @@
 #include "mbedtls/ssl.h"
 #include "ssl_client.h"
 #include "ssl_misc.h"
-#include "mbedtls/debug.h"
+#include "debug_internal.h"
 #include "mbedtls/error.h"
 #include "mbedtls/constant_time.h"