When building against kernel 4.12 and newer, macro name GDT_SIZE used in
(vmmon) include/segs.h collides with macro defined in (kernel source)
arch/x86/include/asm/segment.h, resulting in its redefinition.
To prevent potential problems, rename vmmon's GDT_SIZE to VMMON_GDT_SIZE
and GDT_LIMIT to VMMON_GDT_LIMIT. (There is no GDT_LIMIT in mainline kernel
source but let's be consistent.)