rpki-client: missing initializer in output.c

[Theo Buehler]

Doesn't really matter, but it looks odd and -Wmissing-field-initializers
flags this.

Index: output.c
===================================================================
RCS file: /cvs/src/usr.sbin/rpki-client/output.c,v
retrieving revision 1.27
diff -u -p -r1.27 output.c
--- output.c    30 Aug 2022 18:56:49 -0000      1.27
+++ output.c    4 Nov 2022 12:03:01 -0000
@@ -73,7 +73,7 @@ static const struct outputs {
        { FORMAT_BIRD, "bird", output_bird2 },
        { FORMAT_CSV, "csv", output_csv },
        { FORMAT_JSON, "json", output_json },
-       { 0, NULL }
+       { 0, NULL, NULL }
 };
 
 static FILE    *output_createtmp(char *);