Fix building with glib 2.68:

https://github.com/qtwebkit/qtwebkit/issues/1057

Patch copied from upstream pull request:

https://github.com/qtwebkit/qtwebkit/pull/1058/commits/5b698ba3faffd4e198a45be9fe74f53307395e4b

diff -aurN qtwebkit-5.212.0-alpha4/Source/WTF/wtf/glib/GRefPtr.h qtwebkit-5.212.0-alpha4-mod/Source/WTF/wtf/glib/GRefPtr.h
--- qtwebkit-5.212.0-alpha4/Source/WTF/wtf/glib/GRefPtr.h	2020-03-04 18:16:37.000000000 +0100
+++ qtwebkit-5.212.0-alpha4-mod/Source/WTF/wtf/glib/GRefPtr.h	2021-04-05 06:58:44.763328636 +0200
@@ -29,9 +29,6 @@
 #include <wtf/RefPtr.h>
 #include <algorithm>
 
-extern "C" void g_object_unref(gpointer);
-extern "C" gpointer g_object_ref_sink(gpointer);
-
 namespace WTF {
 
 enum GRefPtrAdoptType { GRefPtrAdopt };