debug: move internal functions declarations to an internal header file

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_client.c b/library/ssl_client.c
index d585ca5..6d988a8 100644
--- a/library/ssl_client.c
+++ b/library/ssl_client.c
@@ -12,7 +12,7 @@
 
 #include <string.h>
 
-#include "mbedtls/debug.h"
+#include "debug_internal.h"
 #include "mbedtls/error.h"
 #include "mbedtls/platform.h"