slideshow: Dont crash when -t is used

Change-Id: I68764b86ad05afb3853ed753a28b4a53580cb68c
diff --git a/slideshow/slideshow.cpp b/slideshow/slideshow.cpp
index 318d805..bc2dcb0 100644
--- a/slideshow/slideshow.cpp
+++ b/slideshow/slideshow.cpp
@@ -87,7 +87,7 @@
     long int timeout = NEXT_TIMEOUT_MS;
     time_t start;
 
-    while ((opt = getopt(argc, argv, "t")) != -1) {
+    while ((opt = getopt(argc, argv, "t:")) != -1) {
         switch (opt) {
         case 't':
             timeout = strtol(optarg, NULL, 0);