Fixed handling of "//" in filenames in verilog pre-processor

frontends/verilog/preproc.cc

@@ -201,7 +201,7 @@ static void input_file(std::istream &f, std::string filename)
 	insert_input("");
 	auto it = input_buffer.begin();
 
-	input_buffer.insert(it, "`file_push " + filename + "\n");
+	input_buffer.insert(it, "`file_push \"" + filename + "\"\n");
 	while ((rc = readsome(f, buffer, sizeof(buffer)-1)) > 0) {
 		buffer[rc] = 0;
 		input_buffer.insert(it, buffer);