Add #ifdef option-test wrappers around another call to an arm/unwind.c

function which is only compiled-in with certain options.

Why is it always the most trivial part of a big commit that takes 3 tries
to get right?

sys/arm/arm/elf_machdep.c

@@ -337,7 +337,9 @@ int
 elf_cpu_unload_file(linker_file_t lf)
 {
 
+#if defined(DDB) || defined(KDTRACE_HOOKS) || defined(STACK)
 	/* Inform the stack(9) code that this module is gone. */
 	unwind_module_unloaded(lf);
+#endif
 	return (0);
 }