x86: ldt.c declare sys_modify_ldt before they get used

Impact: cleanup

In asm/syscalls.h moved out sys_modify_ldt from CONFIG_X86_32 as it is
common for both 32 and 64 bit.

Signed-off-by: Jaswinder Singh <jaswinder@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>