LibIPC: Mark m_sockfd as maybe_unused on non Serenity platforms

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
 }