libsepol: fixup build with gcc-7

... by ignoring a newly detected warning.

pkgs/os-specific/linux/libsepol/default.nix

···
     makeFlagsArray+=("DESTDIR=$out")
   '';
 
+  NIX_CFLAGS_COMPILE = [ "-Wno-error=implicit-fallthrough" ];
+
   passthru = { inherit se_release se_url; };
 
   meta = with stdenv.lib; {