Hack to get posix build partially working

Created a dummy build_git_version.h so I did not have to figure out
how to generate it.

Commented out topic_listener.cpp from src/systemcmds/CMakeLists.txt
because it depends on a generated cpp file.

Link doesn't work mut many modules build

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/posix/drivers/airspeedsim/meas_airspeed_sim.cpp

@@ -589,7 +589,7 @@ measairspeedsim_main(int argc, char *argv[])
 		}
 	}
 
-	int ret;
+	int ret = 0;
 
 	/*
 	 * Start/load the driver.