Fix compilation of D8 with console=readline

BUG=
TEST=

Review URL: http://codereview.chromium.org//6901027

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7680 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/d8-readline.cc

@@ -30,6 +30,8 @@
 #include <readline/readline.h> // NOLINT
 #include <readline/history.h> // NOLINT
 
+// The readline includes leaves RETURN defined which breaks V8 compilation.
+#undef RETURN
 
 #include "d8.h"