Abstracts away time()/stdlib.h into platform

Substitutes time() into a configurable platform interface to allow it to be
easily substituted.
diff --git a/include/mbedtls/ssl_cache.h b/include/mbedtls/ssl_cache.h
index 1155924..3734bb7 100644
--- a/include/mbedtls/ssl_cache.h
+++ b/include/mbedtls/ssl_cache.h
@@ -60,7 +60,7 @@
 struct mbedtls_ssl_cache_entry
 {
 #if defined(MBEDTLS_HAVE_TIME)
-    time_t timestamp;           /*!< entry timestamp    */
+    mbedtls_time_t timestamp;           /*!< entry timestamp    */
 #endif
     mbedtls_ssl_session session;        /*!< entry session      */
 #if defined(MBEDTLS_X509_CRT_PARSE_C)