Fix build with -fno-common

Index: src/rmff.h
--- src/rmff.h.orig
+++ src/rmff.h
@@ -49,7 +49,7 @@ enum {
     PN_SAVE_ENABLED         = 0x01,
     PN_PERFECT_PLAY_ENABLED = 0x02,
     PN_LIVE_BROADCAST       = 0x04,
-} prop_flags;
+};