NFSv4: Ensure nfs4_close_context() is declared as static

Fix another 'sparse' warning in fs/nfs/nfs4proc.c
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4proc.c

@@ -1987,7 +1987,7 @@ out_drop:
 	return 0;
 }
 
-void nfs4_close_context(struct nfs_open_context *ctx, int is_sync)
+static void nfs4_close_context(struct nfs_open_context *ctx, int is_sync)
 {
 	if (ctx->state == NULL)
 		return;