.gitignore: ignore *.tar

Running make tar-pkg results in following:

 # Untracked files:
 #   (use "git add <file>..." to include in what will be committed)
 #
 #       linux-4.0.0-rc3-next-20150313-150225--x86.tar

This patch makes git ignore *.tar files.
Running 'git ls-files -i --exclude-standard' does not show any
tar files excluded from tracking after the change.

Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Boaz Harrosh <boaz@plexistor.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>