pkgsStatic.pixman: fix build

We can only separate debug info from dynamic libraries and
executables, and a static build of pixman doesn't produce either, and
that means the debug output won't be created and the build would fail.

pkgs/development/libraries/pixman/default.nix

···
     sha256 = "0l0m48lnmdlmnaxn2021qi5cj366d9fzfjxkqgcj9bs14pxbgaw4";
   };
 
-  separateDebugInfo = true;
+  separateDebugInfo = !stdenv.hostPlatform.isStatic;
 
   nativeBuildInputs = [ pkg-config ];