Fix format-security violation with croak

Missed one in https://github.com/prusa3d/slic3r/pull/802.

xs/xsp/GCode.xsp

@@ -31,7 +31,7 @@
             try {
                 THIS->do_export(print, path, preview_data);
             } catch (std::exception& e) {
-                croak(e.what());
+                croak("%s\n", e.what());
             }
         %};