SvREFCNT_dec_NN not available until F-20

Copy workaround from Coro/State.xs

--- EV/EV.xs
+++ EV/EV.xs
@@ -9,6 +9,10 @@
 #include "EVAPI.h"
 #include "../Coro/CoroAPI.h"
 
+#ifndef SvREFCNT_dec_NN
+  #define SvREFCNT_dec_NN(sv) SvREFCNT_dec (sv)
+#endif
+
 static struct ev_prepare scheduler;
 static struct ev_idle idler;
 static int inhibit;