debug: move internal functions declarations to an internal header file

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 8c1e372..bd1380a 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -20,7 +20,7 @@
 #include "ssl_debug_helpers.h"
 #include "ssl_misc.h"
 
-#include "mbedtls/debug.h"
+#include "debug_internal.h"
 #include "mbedtls/error.h"
 #include "mbedtls/platform_util.h"
 #include "mbedtls/version.h"