debug: move internal functions declarations to an internal header file

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/debug.c b/library/debug.c
index a9d58e5..c36ed3c 100644
--- a/library/debug.c
+++ b/library/debug.c
@@ -11,7 +11,7 @@
 
 #include "mbedtls/platform.h"
 
-#include "mbedtls/debug.h"
+#include "debug_internal.h"
 #include "mbedtls/error.h"
 
 #include <stdarg.h>