staging: lustre: obdclass: obd_mount: Declare function as static

Declare lustre_mount() function static since it is defined and called in this
file only.

This fixes the following sparse warning:
drivers/staging/lustre/lustre/obdclass/obd_mount.c:1169:15: warning:
symbol 'lustre_mount' was not declared. Should it be static?

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>