Add cdefs decoration to list.h

In case someone includes this from C++ code.

Change-Id: I3b5a8bde5b5b9e8dcee64abaf33383f21eff7046
diff --git a/include/cutils/list.h b/include/cutils/list.h
index eb5a3c8..8190219 100644
--- a/include/cutils/list.h
+++ b/include/cutils/list.h
@@ -18,6 +18,9 @@
 #define _CUTILS_LIST_H_
 
 #include <stddef.h>
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
 
 struct listnode
 {
@@ -48,4 +51,6 @@
 #define list_head(list) ((list)->next)
 #define list_tail(list) ((list)->prev)
 
+__END_DECLS
+
 #endif