Replace cases of time_t with mbedtls_time_t

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 37e4259..75c59a9 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -3178,7 +3178,7 @@
     start = MBEDTLS_GET_UINT64_BE(p, 0);
     p += 8;
 
-    session->start = (time_t) start;
+    session->start = (mbedtls_time_t) start;
 #endif /* MBEDTLS_HAVE_TIME */
 
     /*