SkPDF: fix PDFA-only metadata error

Review-Url: https://codereview.chromium.org/2119883002
diff --git a/src/pdf/SkPDFMetadata.cpp b/src/pdf/SkPDFMetadata.cpp
index 18c8e29..2592496 100644
--- a/src/pdf/SkPDFMetadata.cpp
+++ b/src/pdf/SkPDFMetadata.cpp
@@ -314,7 +314,7 @@
                                     "</pdf:Keywords>\n");
     // TODO: in theory, keywords can be a list too.
 
-    SkString producer("<pdf:Producer>SKPDF_PRODUCER</pdf:Producer>\n");
+    SkString producer("<pdf:Producer>" SKPDF_PRODUCER "</pdf:Producer>\n");
     if (!metadata.fProducer.isEmpty()) {
         // TODO: register a developer prefix to make
         // <skia:SKPDF_CUSTOM_PRODUCER_KEY> a real XML tag.