Fix compatibility with Poppler 0.72.

Patch taken from upstream:
https://tug.org/svn/texlive/trunk/Build/source/texk/web2c/xetexdir/pdfimage.cpp?r1=44964&r2=48969&diff_format=u

--- a/texk/web2c/xetexdir/pdfimage.cpp	2017/08/06 07:12:02	44964
+++ b/texk/web2c/xetexdir/pdfimage.cpp	2018/10/22 04:01:42	48969
@@ -82,19 +82,19 @@
 	switch (pdf_box) {
 		default:
 		case pdfbox_crop:
-			r = page->getCropBox();
+			r = (PDFRectangle *)page->getCropBox();
 			break;
 		case pdfbox_media:
-			r = page->getMediaBox();
+			r = (PDFRectangle *)page->getMediaBox();
 			break;
 		case pdfbox_bleed:
-			r = page->getBleedBox();
+			r = (PDFRectangle *)page->getBleedBox();
 			break;
 		case pdfbox_trim:
-			r = page->getTrimBox();
+			r = (PDFRectangle *)page->getTrimBox();
 			break;
 		case pdfbox_art:
-			r = page->getArtBox();
+			r = (PDFRectangle *)page->getArtBox();
 			break;
 	}