modify_ldt03: Fix compilation.

Fix compilation when struct user_desc is not defined in asm/ldt.h.

NOTE: It would be better to move the user_desc fallback definition from
      modify_ldt01.c to a header in lapi/ and use that instead of the
      hardcoded definition.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>