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.comSigned-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>

include/uapi/linux/coff.h

@@ -11,6 +11,9 @@
    more information about COFF, then O'Reilly has a very excellent book.
 */
 
+#ifndef _UAPI_LINUX_COFF_H
+#define _UAPI_LINUX_COFF_H
+
 #define  E_SYMNMLEN  8   /* Number of characters in a symbol name         */
 #define  E_FILNMLEN 14   /* Number of characters in a file name           */
 #define  E_DIMNUM    4   /* Number of array dimensions in auxiliary entry */
@@ -350,3 +353,5 @@ struct COFF_reloc {
 
 /* For new sections we haven't heard of before */
 #define COFF_DEF_SECTION_ALIGNMENT       4
+
+#endif /* _UAPI_LINUX_COFF_H */