riscv: fix warning in arch/riscv/include/asm/module.h

Fixes warning: 'struct module' declared inside parameter list will not be
visible outside of this definition or declaration

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/module.h

@@ -8,6 +8,7 @@
 
 #define MODULE_ARCH_VERMAGIC    "riscv"
 
+struct module;
 u64 module_emit_got_entry(struct module *mod, u64 val);
 u64 module_emit_plt_entry(struct module *mod, u64 val);