Fix compilation error caused by missing declaration of cupsBackChannelRead

[mjdominus]

Building lw no longer succeeds; it says

g++ -DHAVE_CONFIG_H -I. -I../../src -I../common    -O2 -Wall -Wno-unknown-pragmas   -MT CupsPrintEnvironment.o -MD -MP -MF .deps/CupsPrintEnvironment.Tpo -c -o CupsPrintEnvironment.o `test -f '../common/CupsPrintEnvironment.cpp' || echo './'`../common/CupsPrintEnvironment.cpp
../common/CupsPrintEnvironment.cpp: In member function ‘virtual void DymoPrinterDriver::CCupsPrintEnvironmentForLM::ReadData(DymoPrinterDriver::buffer_t&)’:
../common/CupsPrintEnvironment.cpp:139:23: error: ‘cupsBackChannelRead’ was not declared in this scope
  139 |   ssize_t bytesRead = cupsBackChannelRead((char*)buf, sizeof(buf), 2.5);
      |                       ^~~~~~~~~~~~~~~~~~~
make[4]: *** [Makefile:494: CupsPrintEnvironment.o] Error 1

Maybe cupsBackChannelRead is no longer declared in <cups/cups.h>? Anyway with this change the compilation succeeds.

[hbggbh]

This worked for me! I was trying to get a 450 Twin Turbo working on Ubuntu 22.04.1 running CUPS 2.4.1 and this did the trick.