build: Silence gcc warnings on duplicate const
    
    It looks like we intended to have a const pointer to const contents,
    where the second 'const' was misplaced instead of duplicated. Newer
    gcc now flags the duplication:
    
    ncftool.c:96:33: warning: duplicate 'const' declaration specifier [-Wduplicate-decl-specifier]
     static const struct command_def const *commands[];
                                     ^~~~~
    ncftool.c:676:33: warning: duplicate 'const' declaration specifier [-Wduplicate-decl-specifier]
     static const struct command_def const *commands[] = {
                                     ^~~~~
    
    Signed-off-by: Eric Blake <eblake@redhat.com>
    Reviewed-by: Laine Stump <laine@laine.org>