doc/Makefile: Improve clean target

Delete generated files - sync with .gitignore.

Link: https://lore.kernel.org/ltp/20250206153435.1602181-3-pvorel@suse.cz/
Fixes: 4a72aad ("New LTP documentation")
Acked-by: Andrea Cervesato <andrea.cervesato@suse.com>
Signed-off-by: Petr Vorel <pvorel@suse.cz>

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