Annotate unused argument in mark_as_used() to avoid -Wextra compiler noise

include/libunwind_i.h: In function 'mark_as_used':
include/libunwind_i.h:187:39: warning: unused parameter 'v' [-Wunused-parameter]
diff --git a/include/libunwind_i.h b/include/libunwind_i.h
index a395d68..bdd3f46 100644
--- a/include/libunwind_i.h
+++ b/include/libunwind_i.h
@@ -184,7 +184,7 @@
 
 /* Silence compiler warnings about variables which are used only if libunwind
    is configured in a certain way */
-static inline void mark_as_used(void *v) {
+static inline void mark_as_used(void *v UNUSED) {
 }
 
 #if defined(CONFIG_BLOCK_SIGNALS)