LibIPC: Mark m_sockfd as maybe_unused on non Serenity platforms

Author: https://github.com/davidot
Commit: f151a0d720
Pull-request: https://github.com/SerenityOS/serenity/pull/11123

Userland/Libraries/LibIPC/Decoder.cpp

@@ -167,6 +167,7 @@ ErrorOr<void> Decoder::decode([[maybe_unused]] File& file)
     file = File(fd, File::ConstructWithReceivedFileDescriptor);
     return {};
 #else
+    [[maybe_unused]] auto fd = m_sockfd;
     return Error::from_string_literal("File descriptor passing not supported on this platform");
 #endif
 }