postgresqlPackages.system_stats: fix build on darwin

The build was failing with:

darwin/io_analysis.c:71:21: error: variable length array used
[-Werror,-Wvla]
                        char device_name[kMaxDiskNameSize];
                                         ^~~~~~~~~~~~~~~~

Author: wolfgangwalther

pkgs/servers/sql/postgresql/ext/system_stats.nix

@@ -17,6 +17,8 @@ stdenv.mkDerivation rec {
     hash = "sha256-/xXnui0S0ZjRw7P8kMAgttHVv8T41aOhM3pM8P0OTig=";
   };
 
+  buildFlags = [ "PG_CFLAGS=-Wno-error=vla" ];
+
   installPhase = ''
     runHook preInstall