Fix build with poppler-26.02.0.

Upstream commit 831295894d225b7d6098a5b8ccb51d33da52ef0c.

Index: frmts/pdf/pdfio.cpp
--- frmts/pdf/pdfio.cpp.orig
+++ frmts/pdf/pdfio.cpp
@@ -68,11 +68,20 @@ VSIPDFFileStream::~VSIPDFFileStream()
 /*                                  copy()                              */
 /************************************************************************/
 
+#if POPPLER_MAJOR_VERSION > 26 ||                                              \
+    (POPPLER_MAJOR_VERSION == 26 && POPPLER_MINOR_VERSION >= 2)
+std::unique_ptr<BaseStream> VSIPDFFileStream::copy()
+{
+    return std::make_unique<VSIPDFFileStream>(poParent, nStart, bLimited,
+                                              nLength, dict.copy());
+}
+#else
 BaseStream *VSIPDFFileStream::copy()
 {
     return new VSIPDFFileStream(poParent, nStart, bLimited, nLength,
                                 dict.copy());
 }
+#endif
 
 /************************************************************************/
 /*                             makeSubStream()                          */
