tools/gpio: Enable compiler optimization to catch more bugs

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

tools/gpio/Makefile

 CC = $(CROSS_COMPILE)gcc
-CFLAGS += -Wall -g -D_GNU_SOURCE
+CFLAGS += -O2 -Wall -g -D_GNU_SOURCE
 
 all: lsgpio