[IA64] remove BUILD_BUG_ON from paravirt_getreg()

CC      arch/ia64/kernel/asm-offsets.s
In file included from include/linux/bitops.h:17,
                 from include/linux/kernel.h:15,
                 from include/linux/sched.h:52,
                 from arch/ia64/kernel/asm-offsets.c:9:
arch/ia64/include/asm/bitops.h: In function 'set_bit':
arch/ia64/include/asm/bitops.h:47: error: implicit declaration of function 'BUILD_BUG_ON'

Obvious inclusion of kernel.h doesn't fix it, because of circular dependencies
involving fls.h and log2(). Fixing the latter requires some serious header surgery,
it seems, so just remove BUILD_BUG_ON for now.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/include/asm/paravirt_privop.h

@@ -83,7 +83,6 @@ extern unsigned long ia64_native_getreg_func(int regnum);
 #define paravirt_getreg(reg)					\
 	({							\
 		unsigned long res;				\
-		BUILD_BUG_ON(!__builtin_constant_p(reg));	\
 		if ((reg) == _IA64_REG_IP)			\
 			res = ia64_native_getreg(_IA64_REG_IP); \
 		else						\