lm32: Don't try to build kernel/gpio.o

It was moved in d256a10 to platforms/milkymist

arch/lm32/kernel/Makefile

@@ -5,7 +5,7 @@
 extra-y := head.o init_task.o vmlinux.lds
 obj-y := \
 	sys_lm32.o setup.o traps.o signal.o time.o \
-	ptrace.o irq.o process.o entry.o gpio.o \
+	ptrace.o irq.o process.o entry.o \
 	flat.o sys_call_table.o cpuinfo.o prom.o \
 	platform.o