treewide: Lots of meta.hydraPlatforms -> meta.platforms

In all of these files, there is no meta.platforms but only
meta.hydraPlatforms, which doesn't seem to have any purpose except being
inconsistent.

pkgs/development/libraries/silgraphite/default.nix

@@ -16,6 +16,6 @@ stdenv.mkDerivation rec {
   meta = {
     description = "An advanced font engine";
     maintainers = [ stdenv.lib.maintainers.raskin ];
-    hydraPlatforms = stdenv.lib.platforms.linux;
+    platforms = stdenv.lib.platforms.linux;
   };
 }

pkgs/development/libraries/silgraphite/graphite2.nix

@@ -17,6 +17,6 @@ stdenv.mkDerivation rec {
   meta = {
     description = "An advanced font engine";
     maintainers = [ stdenv.lib.maintainers.raskin ];
-    hydraPlatforms = stdenv.lib.platforms.unix;
+    platforms = stdenv.lib.platforms.unix;
   };
 }