qt6: Replace another qvsnprintf with vsnprintf

We're already using it 4 lines above and qvsnprintf is deprecated

qt6/src/poppler-qiodeviceoutstream.cc

diff --git a/qt6/src/poppler-qiodeviceoutstream.cc b/qt6/src/poppler-qiodeviceoutstream.cc
index ba9bc5b..e3286ff 100644
--- a/qt6/src/poppler-qiodeviceoutstream.cc
+++ b/qt6/src/poppler-qiodeviceoutstream.cc
@@ -59,7 +59,7 @@
     va_end(ap_copy);
     *buf_ptr = new char[size];
 
-    return qvsnprintf(*buf_ptr, size, format, ap);
+    return vsnprintf(*buf_ptr, size, format, ap);
 }
 
 void QIODeviceOutStream::printf(const char *format, ...)