4a110a009 [build] .gitignore: add background.tiff (Sjors Provoost)

Pull request description:

On OSX, running `make deploy` results in three files that were not covered by `.gitignore`:

	background.tiff
	background.tiff.png
	background.tiff@2x.png

Tree-SHA512: cee7a6ebbc50f28bf588121902f6026ec08cf6516d9e56734dfc1b2d3f238e2db5bf87411f6a74b8bd06569f792789ac9f9046e6183f63fd675b953f24b060fc

Backport of Core PR11620
https://github.com/bitcoin/bitcoin/pull/11620/files

I don't have a Mac to test make deploy on, but next best thing:

touch background.tiff
touch background.tiff.png
git status
# shows no background.tiff* files