fcgiwrap: don't error on implicit fallthrough

This errored since ca8aa5d, but should be safe.
Warning is left on (instead of completely disabled),
to not hide potential other fallthroughs if someone
was to make changes to fcgiwrap with this package.

pkgs/servers/fcgiwrap/default.nix

···
     sha256 = "07y6s4mm86cv7p1ljz94sxnqa89y9amn3vzwsnbq5hrl4vdy0zac";
   };
 
+  NIX_CFLAGS_COMPILE = "-Wno-error=implicit-fallthrough";
   configureFlags = [ "--with-systemd" "--with-systemdsystemunitdir=$(out)/etc/systemd/system" ];
 
   nativeBuildInputs = [ autoreconfHook pkgconfig ];