py/mkrules.mk: Define "lib" outside conditional block.

"lib" happened to be defined inside block conditional on $(PROG).

py/mkrules.mk

@@ -115,9 +115,6 @@ ifndef DEBUG
 endif
 	$(Q)$(SIZE) $(PROG)
 
-lib: $(OBJ)
-	$(AR) rcs libmicropython.a $^
-
 clean: clean-prog
 clean-prog:
 	$(RM) -f $(PROG)
@@ -126,6 +123,9 @@ clean-prog:
 .PHONY: clean-prog
 endif
 
+lib: $(OBJ)
+	$(AR) rcs libmicropython.a $^
+
 clean:
 	$(RM) -rf $(BUILD)
 .PHONY: clean