nfsd: define exports_proc_ops with CONFIG_PROC_FS

gcc with W=1 and ! CONFIG_PROC_FS
fs/nfsd/nfsctl.c:161:30: error: ‘exports_proc_ops’
  defined but not used [-Werror=unused-const-variable=]
  161 | static const struct proc_ops exports_proc_ops = {
      |                              ^~~~~~~~~~~~~~~~

The only use of exports_proc_ops is when CONFIG_PROC_FS
is defined, so its definition should be likewise conditional.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>