.gitignore: ignore a patches/ directory

Sometimes, it is useful to have a stack of patches while
testing it, or enabling some sort of debug stuff. Add a
patches/ directory to ignore, as this is the format used
by quilt, with helps to use this tool to manage not-ready
to be applied pile of patches.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>