pitivi: unbreak build

[Breakage introduced in commit 5c4e00b6b797c444c0a0ca434c3b6f
("gst_all_1: 1.6.1 -> 1.8.0 (#14628)").]

The build is failing due to a Wayland test/example program
(<gst-plugins-bad>/tests/examples/waylandsink) being built with a gtk3
input without Wayland support (or detection is failing):

main.c:28:2: error: #error "Wayland is not supported in GTK+"

Fix it by explicitly disabling wayland; pass --enable-wayland=no to
configure.

pkgs/top-level/all-packages.nix

···
   pitivi = callPackage ../applications/video/pitivi {
     gst = gst_all_1 //
       { gst-plugins-bad = gst_all_1.gst-plugins-bad.overrideDerivation
-          (attrs: { nativeBuildInputs = attrs.nativeBuildInputs ++ [ gtk3 ]; });
+          (attrs: { nativeBuildInputs = attrs.nativeBuildInputs ++ [ gtk3 ];
+                    # Fix this build error in ./tests/examples/waylandsink:
+                    #   main.c:28:2: error: #error "Wayland is not supported in GTK+"
+                    configureFlags = attrs.configureFlags or "" + "--enable-wayland=no";
+                  });
       };
   };