NFS: Fix a compile issue in nfs_root

Stephen Rothwell reports:

> /home/test/linux-2.6/fs/nfs/nfsroot.c: In function 'nfs_root_debug':
> /home/test/linux-2.6/fs/nfs/nfsroot.c:110:2: error: 'nfs_debug'
> undeclared (first use in this function)
> /home/test/linux-2.6/fs/nfs/nfsroot.c:110:2: note: each undeclared
> identifier is reported only once for each function it appears in
> make[3]: *** [fs/nfs/nfsroot.o] Error 1
> make[2]: *** [fs/nfs] Error 2
> make[1]: *** [fs] Error 2
> make: *** [sub-make] Error 2

Which is caused by commit 306a0753
(NFS: Allow NFSROOT debugging messages to be enabled dynamically)

Fix is to disable this code when RPC_DEBUG is disabled.

Reported-by: Zimny Lech <napohybelskurwysynom2010@gmail.com>
Tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>