add another unused-var marker in backtrace.c for 14988

changes/bug14988

@@ -0,0 +1,4 @@
+  o Minor bugfixes (compilation):
+    - Fix a compilation warning on s390. Fixes bug 14988; bugfix on
+      0.2.5.2-alpha.
+

src/common/backtrace.c

@@ -80,6 +80,7 @@ clean_backtrace(void **stack, int depth, const ucontext_t *ctx)
 #else
   (void) depth;
   (void) ctx;
+  (void) stack;
 #endif
 }