Tell the compiler that the inline assembler statement
references lguest_entry.

This fixes compile problems with LTO where the variable
and the assembler code may end up in different files.

Cc: x86@kernel.org
Cc: rusty@rustcorp.com.au
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>