Rename _wrap headers to _internal

Makes it clearer that the user is not supposed to include them
diff --git a/library/md.c b/library/md.c
index 5fee809..8359d05 100644
--- a/library/md.c
+++ b/library/md.c
@@ -33,7 +33,7 @@
 #if defined(MBEDTLS_MD_C)
 
 #include "mbedtls/md.h"
-#include "mbedtls/md_wrap.h"
+#include "mbedtls/md_internal.h"
 
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"