lightdm-gtk-greeter: fix build by ignoring a warning

(cherry picked from commit 0479e913094af56ae270a0adcbbe027bfe27488e)

pkgs/applications/display-managers/lightdm-gtk-greeter/default.nix

···
     "--sysconfdir=/etc"
   ] ++ stdenv.lib.optional useGTK2 "--with-gtk2";
 
+  NIX_CFLAGS_COMPILE = [ "-Wno-error=deprecated-declarations" ];
+
   installFlags = [
     "localstatedir=\${TMPDIR}"
     "sysconfdir=\${out}/etc"