[NFC][libc++] Suppress "warning: ignoring return value"

According to the comment on the next line
it's expected behaviour.

GitOrigin-RevId: 3744ba24dcf3ff5ecfa76fade18549528fe49942
diff --git a/include/new b/include/new
index 0562cef..21bc30b 100644
--- a/include/new
+++ b/include/new
@@ -314,7 +314,7 @@
   return ::_aligned_malloc(__size, __alignment);
 #else
   void* __result = nullptr;
-  ::posix_memalign(&__result, __alignment, __size);
+  (void)::posix_memalign(&__result, __alignment, __size);
   // If posix_memalign fails, __result is unmodified so we still return `nullptr`.
   return __result;
 #endif