NFSD: Use "depends on" for PROC_FS dependency

Recently, commit 440bcc59 added a reverse dependency to fs/Kconfig to
ensure that PROC_FS was enabled if NFSD_V4 was enabled.

There is a guideline in Documentation/kbuild/kconfig-language.txt that
states "In general use select only for non-visible symbols (no prompts
anywhere) and for symbols with no dependencies."

A quick grep around other Kconfig files reveals that no entry currently
uses "select PROC_FS" -- every one uses "depends on".  Thus CONFIG_NFSD_V4
should use "depends on PROC_FS" as well.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>