Explicitly strip the solo5 library

The server7 hydra is producing a different binary than my laptop.

packages/solo5/default.nix

@@ -43,6 +43,8 @@ in stdenv.mkDerivation {
     runHook postInstall
   '';
 
+  preFixup = "$STRIP $out/lib/solo5-bindings-genode/solo5.lib.so";
+
   meta = with stdenv.lib; {
     description = "Sandboxed execution environment.";
     homepage = "https://github.com/solo5/solo5";