examples/testd.c: As the C POSIX library recommends include the <unistd.h>
instead of <sys/unistd.h>. This removes an error when building libdaemon
against the musl C library.

Signed-off-by: Jörg Krause <jkrause@posteo.de>
--- a/examples/testd.c	2014-08-30 00:27:08.359832977 +0200
+++ b/examples/testd.c	2014-08-30 00:27:25.439832670 +0200
@@ -23,7 +23,7 @@
 #include <string.h>
 #include <sys/types.h>
 #include <sys/time.h>
-#include <sys/unistd.h>
+#include <unistd.h>
 #include <sys/select.h>
 
 #include <libdaemon/dfork.h>