cinnamon.xapp: actually fix gtk3 module target dir

Before: $out/gtk-3.0/modules/libxapp-gtk3-module.so
After: $out/lib/gtk-3.0/modules/libxapp-gtk3-module.so

cat gtk+-3.0.pc | grep "libdir=" says `libdir=${prefix}/lib`, so install this there.

Another package that installs gtk3 module: libcanberra-gtk3

pkgs/desktops/cinnamon/xapp/default.nix

···
     # Patch pastebin & inxi location
     sed "s|/usr/bin/pastebin|$out/bin/pastebin|" -i scripts/upload-system-info
     sed "s|'inxi'|'${inxi}/bin/inxi'|" -i scripts/upload-system-info
+  '';
 
-    # Patch gtk3 module target dir
-    substituteInPlace libxapp/meson.build \
-         --replace "gtk3_dep.get_pkgconfig_variable('libdir')" "'$out'"
-  '';
+  # Fix gtk3 module target dir. Proper upstream solution should be using define_variable.
+  PKG_CONFIG_GTK__3_0_LIBDIR = "${placeholder "out"}/lib";
 
   meta = with lib; {
     homepage = "https://github.com/linuxmint/xapp";