linux/coff.h: add include guard

Add a header include guard just in case.

My motivation is to allow Kbuild to detect missing include guard:

https://patchwork.kernel.org/patch/11063011/

Before I enable this checker I want to fix as many headers as possible.

Link: http://lkml.kernel.org/r/20190728154728.11126-1-yamada.masahiro@socionext.com


Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>