gpio: tools: add .gitignore for generated files

Add .gitignore for generated files.
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>
[Dropped include dir]
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

tools/gpio/.gitignore

+gpio-event-mon
+gpio-hammer
+lsgpio
+