glib: Make print scaling getter visible

Prefix poppler_document_get_print_scaling with POPPLER_PUBLIC
in glib/poppler-document.h.
diff --git a/glib/poppler-document.h b/glib/poppler-document.h
index 32614c5..cb289f2 100644
--- a/glib/poppler-document.h
+++ b/glib/poppler-document.h
@@ -373,6 +373,7 @@
 PopplerPDFConformance poppler_document_get_pdf_conformance (PopplerDocument *document);
 POPPLER_PUBLIC
 gchar             *poppler_document_get_metadata           (PopplerDocument *document);
+POPPLER_PUBLIC
 PopplerPrintScaling poppler_document_get_print_scaling     (PopplerDocument *document);
 
 /* Attachments */