tools: Add ifwitool to .gitignore

Follow up fix to the commit

56bf4f86 ("x86: Add ifwitool for Intel Integrated Firmware Image")

in order to ignore created binary.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>