debug.h: allow to #include it more than one time

It's easy to have debug.h #include'd multiple times. While it would be
better to fix that, getting a build failure is nasty. Avoid that.

Signed-off-by: Arnaud Patard <apatard@mandriva.com>
diff --git a/debug.h b/debug.h
index 653da84..52e8e96 100644
--- a/debug.h
+++ b/debug.h
@@ -1,3 +1,6 @@
+#ifndef _DEBUG_H
+#define _DEBUG_H
+
 #include <features.h>
 
 /* debug levels:
@@ -15,3 +18,4 @@
 
 # define debug(level, expr...) debug_(level, __FILE__, __LINE__, expr)
 
+#endif