Fix build with -fno-common: remove unused variable

Index: src/conf.h
--- src/conf.h.orig
+++ src/conf.h
@@ -66,11 +66,6 @@ typedef struct {
 #endif /* HAVE_GTK */
 } Options;
 
-enum {
-	POSTMODE_GUI,
-	POSTMODE_CMD
-} PostmodeType;
-
 typedef struct {
 	char *label;
 	char *command;