CI Improvements

[matt335672]

Added the following to the CI:-

• An extra compiler (CLANG)
• Compiler flags -Wall -Wextra -Werror

The CI has been restructured into three jobs. The first gets the PA sources, and the other two then build the module against these sources. A dependency prevents the compilers running until the sources are available.

Example output from the CI is here:-

https://github.com/matt335672/pulseaudio-module-xrdp/actions/runs/1330330090

The second commit fixes the compiler warnings/errors generated for unused variables:-

 CC       module_xrdp_sink_la-module-xrdp-sink.lo
module-xrdp-sink.c: In function ‘thread_func’:
module-xrdp-sink.c:376:15: error: unused variable ‘now’ [-Werror=unused-variable]
  376 |     pa_usec_t now;
      |               ^~~
module-xrdp-sink.c:375:9: error: unused variable ‘ret’ [-Werror=unused-variable]
  375 |     int ret;
      |         ^~~
cc1: all warnings being treated as errors
make[2]: *** [Makefile:481: module_xrdp_sink_la-module-xrdp-sink.lo] Error 1