Fixes missing dependency in ss.h on platform.h

Fixes #522 - 'mbedtls_time_t does not name a type in ssl.h'
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 82c0760..bc62336 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -52,7 +52,7 @@
 #endif
 
 #if defined(MBEDTLS_HAVE_TIME)
-#include <time.h>
+#include "platform.h"
 #endif
 
 /*