libsepol: fixup build with gcc-7

... by ignoring a newly detected warning.

+2
+2
pkgs/os-specific/linux/libsepol/default.nix
··· 18 makeFlagsArray+=("DESTDIR=$out") 19 ''; 20 21 passthru = { inherit se_release se_url; }; 22 23 meta = with stdenv.lib; {
··· 18 makeFlagsArray+=("DESTDIR=$out") 19 ''; 20 21 + NIX_CFLAGS_COMPILE = [ "-Wno-error=implicit-fallthrough" ]; 22 + 23 passthru = { inherit se_release se_url; }; 24 25 meta = with stdenv.lib; {