minui: add ev_get_epollfd() to retrieve epoll file descriptor

To allow use of ev_* functions with an external main loop.

Change-Id: If73717b64d7c455ca726b90a815a31c1edf52544
diff --git a/minui/events.c b/minui/events.c
index fe364bf..008a7d8 100644
--- a/minui/events.c
+++ b/minui/events.c
@@ -132,6 +132,11 @@
     return ret;
 }
 
+int ev_get_epollfd(void)
+{
+    return epollfd;
+}
+
 void ev_exit(void)
 {
     while (ev_count > 0) {