php: Fix fallout from 1228caae9e5f

Signed-off-by: Austin Seipp <[e-mail]>

pkgs/development/interpreters/php/5.4.nix

···
         configureFlags = ["--enable-fpm"];
       };
 
-      mssql = stdenv.lib.optional (!stdenv.isDarwin) {
+      mssql = stdenv.lib.optionalAttrs (!stdenv.isDarwin) {
         configureFlags = ["--with-mssql=${freetds}"];
         buildInputs = [freetds];
       };