x86/asm: Make boot_gdt_descr local

As far as I can see, it was never used outside of head_32.S. Not even
when added in 2004. So make it local.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
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>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20191003095238.29831-2-jslaby@suse.cz

arch/x86/kernel/head_32.S

@@ -597,8 +597,6 @@ int_msg:
  */
 
 	.data
-.globl boot_gdt_descr
-
 	ALIGN
 # early boot GDT descriptor (must use 1:1 address mapping)
 	.word 0				# 32 bit align gdt_desc.address