x86/boot: Fix incorrect startup_gdt_descr.size

Since the size value is added to the base address to yield the last valid
byte address of the GDT, the current size value of startup_gdt_descr is
incorrect (too large by one), fix it.

[ mingo: This probably never mattered, because startup_gdt[] is only used
         in a very controlled fashion - but make it consistent nevertheless. ]

Fixes: 866b556e

 ("x86/head/64: Install startup GDT")
Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20230807084547.217390-1-ytcoode@gmail.com