MIPS: gitignore: ignore generated .c files

Add ashldi3.c and bswapsi.c to the list of ignored files.
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/16905/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/boot/compressed/.gitignore

+ashldi3.c
+bswapsi.c