Allow to forward declare of public structs #1215

Thanks to forward declare we can declare `struct` in our header file instead making #include
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index ee06b2f..db54c6a 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -87,7 +87,7 @@
  * \brief           Options for RSASSA-PSS signature verification.
  *                  See \c mbedtls_rsa_rsassa_pss_verify_ext()
  */
-typedef struct
+typedef struct mbedtls_pk_rsassa_pss_options
 {
     mbedtls_md_type_t mgf1_hash_id;
     int expected_salt_len;
@@ -107,7 +107,7 @@
 /**
  * \brief           Item to send to the debug module
  */
-typedef struct
+typedef struct mbedtls_pk_debug_item
 {
     mbedtls_pk_debug_type type;
     const char *name;
@@ -125,7 +125,7 @@
 /**
  * \brief           Public key container
  */
-typedef struct
+typedef struct mbedtls_pk_context
 {
     const mbedtls_pk_info_t *   pk_info; /**< Public key informations        */
     void *                      pk_ctx;  /**< Underlying public key context  */