dm: fix unused-private-field error when skia_use_expat=false

Change-Id: I309aa420a7900c048c34873ede618f123719823c
Reviewed-on: https://skia-review.googlesource.com/115928
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index cd4f111..2d8d949 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -2092,6 +2092,7 @@
                                       xmlWriter.get())
                             .get());
 #else
+    (void)fPageIndex;
     return Error("SVG sink is disabled.");
 #endif // SK_XML
 }