i2c: acorn: is tristate and should use module.h

This file is controlled by a tristate Kconfig option, and hence
needs to include module.h so that it can get module_init() once
we relocate it from init.h into module.h in the future.

Note that module_exit() appears to be missing from the driver, so
it is questionable whether it would actually work for a removal
and reload cycle if it was configured for a modular build.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>