Use proper prototypes in struct boot_module_t

[dim]

Summary

With clang 4.0.0, we are getting the following warnings about struct
boot_module_t in efi's boot_module.h:

In file included from sys/boot/efi/boot1/ufs_module.c:41:
sys/boot/efi/boot1/boot_module.h:67:14: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
        void (*init)();
                    ^
                     void
sys/boot/efi/boot1/boot_module.h:92:16: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
        void (*status)();
                      ^
                       void
sys/boot/efi/boot1/boot_module.h:95:24: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
        dev_info_t *(*devices)();
                              ^
                               void
3 errors generated.

Fix this by adding 'void' to the parameter lists. No functional change.