[sysprof] build: fix warning

[Christian Hergert <chergert src gnome org>]

commit dacf2ad2b3266e9877b82d0420fca67b0cba8615
Author: Christian Hergert <chergert redhat com>
Date:   Mon Jul 6 16:08:44 2020 -0700

    build: fix warning

 examples/app.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/examples/app.c b/examples/app.c
index abb45aa..03adf85 100644
--- a/examples/app.c
+++ b/examples/app.c
@@ -5,7 +5,9 @@
  * to the capture from your application.
  */
 
-#define _GNU_SOURCE
+#ifndef _GNU_SOURCE
+# define _GNU_SOURCE
+#endif
 
 #ifdef __linux__
 # include <sched.h>