tools: .gitignore: add asn1_compiler

Add the tool to the ignore list to prevent being marked as unversioned.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

tools/.gitignore

+/asn1_compiler
 /atmel_pmecc_params
 /bin2header
 /bmp_logo