x86: Fix compile error in arch/x86/kernel/early_printk.c

Fix compile problem:

  CC      arch/x86/kernel/early_printk.o
In file included from /home/jeremy/hg/xen/paravirt/linux/arch/x86/kernel/early_printk.c:17:
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h: In function 'pmd_page':
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h:516: error: implicit declaration of function '__pfn_to_section'
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h:516: warning: initialization makes pointer from integer without a cast
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h:516: error: implicit declaration of function '__section_mem_map_addr'
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h:516: warning: return makes pointer from integer without a cast
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h: In function 'pud_page':
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h:586: warning: initialization makes pointer from integer without a cast
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h:586: warning: return makes pointer from integer without a cast
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h: In function 'pgd_page':
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h:625: warning: initialization makes pointer from integer without a cast
/home/jeremy/hg/xen/paravirt/linux/arch/x86/include/asm/pgtable.h:625: warning: return makes pointer from integer without a cast

This is a cycling dependency between asm/pgtable.h and linux/mmzone.h
when using CONFIG_SPARSEMEM.  Rather than hacking up the headers some
more, remove asm/pgtable.h, since early_printk.c doesn't actually need
it.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>