nginx: fixup build with other than gcc11

This parameter isn't known by clang or older gcc,
and that breaks compilation there:
https://hydra.nixos.org/build/174714339
https://hydra.nixos.org/build/174713306

Flag was added in commit 6508304a in PR #170102.

pkgs/servers/http/nginx/generic.nix

@@ -107,6 +107,7 @@ stdenv.mkDerivation {
   NIX_CFLAGS_COMPILE = toString ([
     "-I${libxml2.dev}/include/libxml2"
     "-Wno-error=implicit-fallthrough"
+  ] ++ optionals (stdenv.cc.isGNU && lib.versionAtLeast stdenv.cc.version "11") [
     # fix build vts module on gcc11
     "-Wno-error=stringop-overread"
   ] ++ optional stdenv.isDarwin "-Wno-error=deprecated-declarations");