Makefile: ignore missing docs

e.g. when calling `make clean` out-of-tree

Makefile

@@ -1002,8 +1002,8 @@ clean:
 	rm -rf vloghtb/Makefile vloghtb/refdat vloghtb/rtl vloghtb/scripts vloghtb/spec vloghtb/check_yosys vloghtb/vloghammer_tb.tar.bz2 vloghtb/temp vloghtb/log_test_*
 	rm -f tests/svinterfaces/*.log_stdout tests/svinterfaces/*.log_stderr tests/svinterfaces/dut_result.txt tests/svinterfaces/reference_result.txt tests/svinterfaces/a.out tests/svinterfaces/*_syn.v tests/svinterfaces/*.diff
 	rm -f  tests/tools/cmp_tbdata
-	$(MAKE) -C docs clean
-	$(MAKE) -C docs/images clean
+	-$(MAKE) -C docs clean
+	-$(MAKE) -C docs/images clean
 	rm -rf docs/source/cmd docs/util/__pycache__
 
 clean-abc: