Move padlock.h to library

`padlock.h` is actually an internal header and is moved accordingly.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/library/padlock.c b/library/padlock.c
index 8373374..b8ba105 100644
--- a/library/padlock.c
+++ b/library/padlock.c
@@ -27,7 +27,7 @@
 
 #if defined(MBEDTLS_PADLOCK_C)
 
-#include "mbedtls/padlock.h"
+#include "padlock.h"
 
 #include <string.h>