tools/counter: Add .gitignore

Ignore counter_example and include/linux/counter.h.

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Link: https://lore.kernel.org/r/tencent_3E0341A1C9AC8FCB7915E7D791D117445707@qq.com/


Signed-off-by: William Breathitt Gray <william.gray@linaro.org>