autofs: Only declare function when CONFIG_COMPAT is defined

The patch solves the following warnings message when CONFIG_COMPAT
is not defined:

fs/autofs/root.c:30: warning: ‘autofs_root_compat_ioctl’ declared ‘static’ but never defined

Signed-off-by: Márton Németh <nm127@freemail.hu>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>