tier1: exclude steam deck from KeyValues

tier1/KeyValues.cpp

@@ -2195,6 +2195,9 @@ bool EvaluateConditional( const char *str )
 	if ( *str == '!' )
 		bNot = true;
 
+	if( Q_stristr( str, "$DECK" ) )
+		return false ^ bNot; // Steam deck unsupported
+
 	if ( Q_stristr( str, "$X360" ) )
 		return IsX360() ^ bNot;