Missing forward declarations in DSCs.

Makes gcc5 happy.
diff --git a/contiki/apps/about-dsc.c b/contiki/apps/about-dsc.c
index 9a3fdc4..436d304 100644
--- a/contiki/apps/about-dsc.c
+++ b/contiki/apps/about-dsc.c
@@ -35,6 +35,7 @@
 
 #include "dsc.h"
 
+static const struct ctk_icon about_icon;
 /*-----------------------------------------------------------------------------------*/
 const DSC(about_dsc,
     "About Contiki",