[analyzer] decode() a bytes object to make Python3 happy

The output of subprocess.check_output is decode()'d through the rest of
this python program, but one appears to have been missed for the output
of the call to "clang -print-file-name=include".

On Windows, with Python 3.6, this leads to the 'args' array being a mix of
bytes and strings, which causes exceptions later down the line.

I can't easily test with python2 on Windows, but python2 on Ubuntu 18.04
was happy with this change.