libdockapp: Add #include <time.h> to basic example.

Otherwise, we get the following compiler warning:
basic.c: In function ‘main’:
basic.c:111:2: warning: implicit declaration of function ‘time’ [-Wimplicit-function-declaration]
  srandom(time(NULL));
  ^

libdockapp/examples/basic/basic.c

@@ -9,6 +9,7 @@
 
 /* also includes Xlib, Xresources, XPM, stdlib and stdio */
 #include <dockapp.h>
+#include <time.h>
 
 /* include the pixmap to use */
 #include "ball_red.xpm"