kvm tools: Define __compiletime_error helper

To eliminate compile errors like

 |  CC       builtin-run.o
 | In file included from ../../arch/x86/include/asm/system.h:7:0,
 |                 from include/kvm/barrier.h:13,
 |                 from builtin-run.c:16:
 | ../../arch/x86/include/asm/cmpxchg.h:11:13: error: no previous prototype for ‘__xchg_wrong_size’ [-Werror=missing-prototypes]
 | ../../arch/x86/include/asm/cmpxchg.h: In function ‘__xchg_wrong_size’:
 | ../../arch/x86/include/asm/cmpxchg.h:12:2: error: expected declaration specifiers before ‘__compiletime_error’

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>