build: lib.rules: Add common-clean

During the build reshuffling, I missed this, so doing
'make clean' in a certain src/lib folder doesn't clean
up everything anymore. Fix that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

src/lib.rules

@@ -25,5 +25,5 @@ install-default:
 %: %-default
 	@true
 
-clean:
+clean: common-clean
 	$(Q)rm -f *~ *.o *.d *.gcno *.gcda *.gcov $(ALL)