Make function calls show up in stack traces again

Note that adding --show-trace prevents functions calls from being
tail-recursive, so an expression that evaluates without --show-trace
may fail with a stack overflow if --show-trace is given.

src/libstore/globals.hh

@@ -186,6 +186,9 @@ struct Settings {
     /* Whether to lock the Nix client and worker to the same CPU. */
     bool lockCPU;
 
+    /* Whether to show a stack trace if Nix evaluation fails. */
+    bool showTrace;
+
 private:
     SettingsMap settings, overrides;