More granular define selections within code to allow for smaller code

sizes

include/polarssl/ssl_cache.h

@@ -46,7 +46,9 @@ struct _ssl_cache_entry
 {
     time_t timestamp;           /*!< entry timestamp    */
     ssl_session session;        /*!< entry session      */
+#if defined(POLARSSL_X509_PARSE_C)
     x509_buf peer_cert;         /*!< entry peer_cert    */
+#endif
     ssl_cache_entry *next;      /*!< chain pointer      */
 };