bin/khronos-update: having a folder in include/ is not a requirement

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6004>
diff --git a/bin/khronos-update.py b/bin/khronos-update.py
index f95c895..68801d5 100755
--- a/bin/khronos-update.py
+++ b/bin/khronos-update.py
@@ -197,6 +197,7 @@
             source.sync()
 
         # Make sure all the API files are handled by this script
-        for file in pathlib.Path('include/' + group['inc_folder']).iterdir():
-            if file not in [source.file for source in group['sources']]:
-                error('{} is unknown, please add it to SOURCES'.format(file))
+        if 'inc_folder' in group:
+            for file in pathlib.Path('include/' + group['inc_folder']).iterdir():
+                if file not in [source.file for source in group['sources']]:
+                    error('{} is unknown, please add it to SOURCES'.format(file))