Add build include directory to gtkdoc source paths.

This enables gtkdoc to produce documentation on files that were
generated, using configure_file, for example.

mesonbuild/modules/gnome.py

@@ -702,6 +702,8 @@ class GnomeModule(ExtensionModule):
                 for inc_dir in src_dir.get_incdirs():
                     header_dirs.append(os.path.join(state.environment.get_source_dir(),
                                                     src_dir.get_curdir(), inc_dir))
+                    header_dirs.append(os.path.join(state.environment.get_build_dir(),
+                                                    src_dir.get_curdir(), inc_dir))
             else:
                 header_dirs.append(src_dir)