debug: move internal functions declarations to an internal header file

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index 6579c96..5753cf9 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -18,7 +18,7 @@
 
 #include "mbedtls/ssl.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"