module: Drop redundant declaration of struct module

Struct module is already declared at the beginning of the file, no
need to declare it again.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: 93c2e105

 ("module: Optimize __module_address() using a latched RB-tree")
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Jessica Yu <jeyu@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jessica Yu <jeyu@redhat.com>