x86/boot: Add missing va_end() to die()

Each call to va_start() must have a corresponding call to va_end()
before the end of the function. Add the missing va_end().

Found with Coccinelle.

Signed-off-by: Mattias Jacobsson <2pi@mok.nu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20181128161607.10973-1-2pi@mok.nu