Fix naked call to time() with platform call

In ssl_cache.c a call to time() was being made instead of it's platform
equivalent.
diff --git a/library/ssl_cache.c b/library/ssl_cache.c
index 9b62de2..c771d7f 100644
--- a/library/ssl_cache.c
+++ b/library/ssl_cache.c
@@ -138,7 +138,7 @@
 {
     int ret = 1;
 #if defined(MBEDTLS_HAVE_TIME)
-    mbedtls_time_t t = time( NULL ), oldest = 0;
+    mbedtls_time_t t = mbedtls_time( NULL ), oldest = 0;
     mbedtls_ssl_cache_entry *old = NULL;
 #endif
     mbedtls_ssl_cache_context *cache = (mbedtls_ssl_cache_context *) data;