doc/Makefile: Improve clean target

Delete generated files - sync with .gitignore.

Link: https://lore.kernel.org/ltp/[email protected]/
Fixes: 4a72aad ("New LTP documentation")
Acked-by: Andrea Cervesato <[email protected]>
Signed-off-by: Petr Vorel <[email protected]>

Author: pevik

doc/Makefile

@@ -15,4 +15,4 @@ spelling:
 	sphinx-build -b spelling -d build/doctree . build/spelling
 
 clean:
-	rm -rf html/
+	rm -rf html/ build/ _static/syscalls.rst _static/tests.rst syscalls.tbl