memory: add [[gnu::malloc]] attribute to memalign()

Fedora 31's gcc complains that the attributes on
__libc_memalign() don't match memalign()'s, so humor it.

src/core/memory.cc

@@ -1588,6 +1588,7 @@ int __libc_posix_memalign(void** ptr, size_t align, size_t size) throw ();
 
 extern "C"
 [[gnu::visibility("default")]]
+[[gnu::malloc]]
 void* memalign(size_t align, size_t size) throw () {
     if (try_trigger_error_injector()) {
         return nullptr;