Shrink struct module: CONFIG_UNUSED_SYMBOLS ifdefs

module.c and module.h conatains code for finding
exported symbols which are declared with EXPORT_UNUSED_SYMBOL,
and this code is compiled in even if CONFIG_UNUSED_SYMBOLS is not set
and thus there can be no EXPORT_UNUSED_SYMBOLs in modules anyway
(because EXPORT_UNUSED_SYMBOL(x) are compiled out to nothing then).

This patch adds required #ifdefs.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>