If we don't do that, the traversal of the sources list to generate the
sources file for g-ir-scanner is going to explode with a Python
backtrace like this one:
File "/usr/lib/python3.5/site-packages/mesonbuild/modules/gnome.py", line 463, in generate_gir
gir_filelist.write(os.path.join(srcdir, s) + '\n')
File "/usr/lib/python3.5/posixpath.py", line 89, in join
genericpath._check_arg_types('join', a, *p)
File "/usr/lib/python3.5/genericpath.py", line 143, in _check_arg_types
(funcname, s.__class__.__name__)) from None
if the sources list contains a list.