sh: decompressor: add shipped files to .gitignore

These files are copied from arch/sh/lib, so should be ignored by git.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/sh/boot/compressed/.gitignore

+ashiftrt.S
+ashldi3.c
+ashlsi3.S
+ashrsi3.S
+lshrsi3.S
 vmlinux.bin.*