remove *.so files in make clean

src/Makefile

@@ -64,7 +64,7 @@ tests:
 	$(MAKE) -C tests all
 
 clean:
-	rm -f *.o $(EXECS)
+	rm -f *.o *.so $(EXECS)
 	for i in $(extmod_subdirs); do [ -d $$i ] && $(MAKE) -C $$i clean; done
 	$(MAKE) -C tests clean