Move pk_info structure to its own file
This is in preparation to adding support for opaque keys in the pk layer. The
header files are for use as follows:
* pk.h for users of the pk interface.
* pk_info.h for implementers of opaque key engines.
* pk_internal.h only inside libmbedcrypto to implement built-in pk types.
diff --git a/library/pk.c b/library/pk.c
index b48f4d9..d080c75 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -28,6 +28,7 @@
#if defined(MBEDTLS_PK_C)
#include "mbedtls/pk.h"
#include "mbedtls/pk_internal.h"
+#include "mbedtls/pk_info.h"
#if defined(MBEDTLS_RSA_C)
#include "mbedtls/rsa.h"