efi_loader: documentation codepage_437[]

Variables cannot be documented via kernel-doc. Avoid 'make htmldocs' build
warning

    ./include/charset.h:19: warning:
    cannot understand function prototype: 'const u16 codepage_437[128]; '

Fixes: 70616a1e ("efi_loader: move codepage 437 table")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>