Unify the declarations of PSOutPaperSize

The clazy CI complains about a 'struct vs. class' inconsistency.
diff --git a/poppler/PSOutputDev.h b/poppler/PSOutputDev.h
index 119a1d6..c667020 100644
--- a/poppler/PSOutputDev.h
+++ b/poppler/PSOutputDev.h
@@ -61,7 +61,7 @@
 struct PSFont8Info;
 struct PSFont16Enc;
 class PSOutCustomColor;
-class PSOutPaperSize;
+struct PSOutPaperSize;
 class PSOutputDev;
 
 //------------------------------------------------------------------------