Fix warnings from mingw64 in timing.c
Backport from dda52139 from the 1.3 branch
diff --git a/include/polarssl/timing.h b/include/polarssl/timing.h
index 1634774..2f59f0c 100644
--- a/include/polarssl/timing.h
+++ b/include/polarssl/timing.h
@@ -55,6 +55,10 @@
* \brief Setup an alarm clock
*
* \param seconds delay before the "alarmed" flag is set
+ *
+ * \warning Only one alarm at a time is supported. In a threaded
+ * context, this means one for the whole process, not one per
+ * thread.
*/
void set_alarm( int seconds );