gnome: Flatten the sources for generate_gir()

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.

mesonbuild/modules/gnome.py

@@ -423,7 +423,7 @@ class GnomeModule(ExtensionModule):
             raise MesonException('gobject-introspection dependency was not found, gir cannot be generated.')
         ns = kwargs.pop('namespace')
         nsversion = kwargs.pop('nsversion')
-        libsources = kwargs.pop('sources')
+        libsources = mesonlib.flatten(kwargs.pop('sources'))
         girfile = '%s-%s.gir' % (ns, nsversion)
         srcdir = os.path.join(state.environment.get_source_dir(), state.subdir)
         builddir = os.path.join(state.environment.get_build_dir(), state.subdir)