NFSv4: Fix warning "no previous prototype for ‘nfs4_listxattr’"

Make it static

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index d30f88c..4b1491d 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -8848,7 +8848,7 @@
 #endif
 };
 
-ssize_t nfs4_listxattr(struct dentry *dentry, char *list, size_t size)
+static ssize_t nfs4_listxattr(struct dentry *dentry, char *list, size_t size)
 {
 	ssize_t error, error2;