silgraphite: fix evaluation errors on Darwin

pkgs/development/libraries/silgraphite/default.nix

···
 stdenv.mkDerivation rec {
   version = "2.3.1";
   name = "silgraphite-2.3.1";
-  
+
   src = fetchurl {
     url = "mirror://sourceforge/silgraphite/silgraphite/${version}/${name}.tar.gz";
     sha256 = "9b07c6e91108b1fa87411af4a57e25522784cfea0deb79b34ced608444f2ed65";
···
   meta = {
     description = "An advanced font engine";
     maintainers = [ stdenv.lib.maintainers.raskin ];
-    platforms = stdenv.lib.platforms.linux;
+    hydraPlatforms = stdenv.lib.platforms.linux;
   };
 }

pkgs/development/libraries/silgraphite/graphite2.nix

···
   meta = {
     description = "An advanced font engine";
     maintainers = [ stdenv.lib.maintainers.raskin ];
-    platforms = stdenv.lib.platforms.linux;
+    hydraPlatforms = stdenv.lib.platforms.linux;
   };
 }