ARM: 7992/1: boot: compressed: ignore bswapsdi2.S

Commit 017f161a (ARM: 7877/1: use built-in byte swap function) added
bswapsdi2.{o,S} to arch/arm/boot/compressed/Makefile, but didn't update
the .gitignore. Thus after a a build git status shows bswapsdi2.S as a
new file, which is a little annoying.

This patch updates arch/arm/boot/compressed/.gitignore to ignore
bswapsdi2.S, as we already do for ashldi3.S and others.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Kim Phillips <kim.phillips@freescale.com>
Cc: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>