fix the encoding problem for subprocess.check_output

[stmoon]

Describe problem solved by this pull request
When I compile make px4_sitl_rtps, I found the error like below with just standalone, no ROS.

stmoon@stmoon-XPS-13-9370:/home/px4/firmware.rtps/Firmware.stmoon/build/px4_sitl_rtps/src/modules/micrortps_bridge/micrortps_client/micrortps_agent/build$ make 
Scanning dependencies of target micrortps_agent
[  0%] Building CXX object CMakeFiles/micrortps_agent.dir/RtpsTopics.cpp.o
In file included from /home/px4/firmware.rtps/Firmware.stmoon/build/px4_sitl_rtps/src/modules/micrortps_bridge/micrortps_client/micrortps_agent/RtpsTopics.h:38:0,
                 from /home/px4/firmware.rtps/Firmware.stmoon/build/px4_sitl_rtps/src/modules/micrortps_bridge/micrortps_client/micrortps_agent/RtpsTopics.cpp:34:
/home/px4/firmware.rtps/Firmware.stmoon/build/px4_sitl_rtps/src/modules/micrortps_bridge/micrortps_client/micrortps_agent/adc_report_Publisher.h:60:18: error: ‘px4’ has not been declared
     void publish(px4::msg::adc_report* st);
                  ^~~
/home/px4/firmware.rtps/Firmware.stmoon/build/px4_sitl_rtps/src/modules/micrortps_bridge/micrortps_client/micrortps_agent/adc_report_Publisher.h:60:38: error: expected ‘,’ or ‘...’ before ‘*’ token

Describe your solution

I found the python code output type of subprocess.check_output is binary in my case. To clarify it, I clarify the encoding type, utf-8 like below

rosversion_out = rosversion_out.rstrip().decode('utf-8')

Actually, I solved the problem in another pull request. However, I think the problem should be separated.