LibIPC: Fix 'unused private member' error when building with clang

This was breaking the Lagom build when using clang, as m_sockfd's only
use is behind an #ifdef __serenity__. (void) it elsewhere to fix that.

Libraries/LibIPC/Decoder.cpp

@@ -178,6 +178,7 @@ bool Decoder::decode(File& file)
     return true;
 #else
     (void)file;
+    (void)m_sockfd;
     warnln("fd passing is not supported on this platform, sorry :(");
     return false;
 #endif