tools: gpio: Fix out-of-tree build regression

Commit 0161a94e ("tools: gpio: Correctly add make dependencies for
gpio_utils") added a make rule for gpio-utils-in.o but used $(output)
instead of the correct $(OUTPUT) for the output directory, breaking
out-of-tree build (O=xx) with the following error:

  No rule to make target 'out/tools/gpio/gpio-utils-in.o', needed by 'out/tools/gpio/lsgpio-in.o'.  Stop.

Fix that.

Fixes: 0161a94e ("tools: gpio: Correctly add make dependencies for gpio_utils")
Cc: <stable@vger.kernel.org>
Cc: Laura Abbott <labbott@redhat.com>
Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Link: https://lore.kernel.org/r/20200325103154.32235-1-anssi.hannula@bitwise.fiReviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

tools/gpio/Makefile

@@ -35,7 +35,7 @@ $(OUTPUT)include/linux/gpio.h: ../../include/uapi/linux/gpio.h
 
 prepare: $(OUTPUT)include/linux/gpio.h
 
-GPIO_UTILS_IN := $(output)gpio-utils-in.o
+GPIO_UTILS_IN := $(OUTPUT)gpio-utils-in.o
 $(GPIO_UTILS_IN): prepare FORCE
 	$(Q)$(MAKE) $(build)=gpio-utils