ARM: module: fix modsign build error

The asm/module.h header file can not be included standalone, which
breaks the module signing code after a recent change:

In file included from kernel/module-internal.h:13,
                 from kernel/module_signing.c:17:
arch/arm/include/asm/module.h:37:27: error: 'struct module' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
 u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val);

This adds a forward declaration of struct module to make it all work.

Fixes: f314dfea ("modsign: log module name in the event of an error")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jessica Yu <jeyu@kernel.org>