x86: use BUILD_BUG_ON() for the size of struct intel_mp_floating

Use BUILD_BUG_ON() instead of compile-time error technique with
extern non-exsistent function.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>