Add fit-dtb.blob* to .gitignore.

Support for compressed fit-dtb.blob was added in:

  commit 95f4bbd5 ("lib: fdt: Allow LZO and GZIP DT compression in
  U-Boot")

Adjust .gitignore to also exclude compressed blobs.

Signed-off-by: Vagrant Cascadian <vagrant@reproducible-builds.org>