.gitignore: Add an entry for rtc-range

The rtc-range binary should also be ignored by git.

Add an entry for it.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211203190324.1514671-1-festevam@gmail.com