detect: fix decoder only events

Add missing setup part of the decoder event sgh.

Bug: OISF#7414.

src/detect-engine-build.c

@@ -2029,6 +2029,7 @@ int SigPrepareStage4(DetectEngineCtx *de_ctx)
     if (de_ctx->decoder_event_sgh != NULL) {
         /* no need to set filestore count here as that would make a
          * signature not decode event only. */
+        SigGroupHeadBuildNonPrefilterArray(de_ctx, de_ctx->decoder_event_sgh);
     }
 
     int dump_grouping = 0;