xfs, proc: hide unused xfs procfs helpers

These two functions now trigger a warning when CONFIG_PROC_FS is disabled:

fs/xfs/xfs_stats.c:128:12: error: 'xqmstat_proc_show' defined but not used [-Werror=unused-function]
 static int xqmstat_proc_show(struct seq_file *m, void *v)
            ^~~~~~~~~~~~~~~~~
fs/xfs/xfs_stats.c:118:12: error: 'xqm_proc_show' defined but not used [-Werror=unused-function]
 static int xqm_proc_show(struct seq_file *m, void *v)
            ^~~~~~~~~~~~~

Previously, they were referenced from an unused 'static const' structure,
which is silently dropped by gcc.

We can address the warning by adding the same #ifdef around them that
hides the reference.

Fixes: 3f3942ac ("proc: introduce proc_create_single{,_data}")
Cc: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>