x86: early_printk.c - fix pgtable.h unification fallout

 arch/x86/kernel/early_printk.c: In function ‘early_dbgp_init’:
 arch/x86/kernel/early_printk.c:827: error: ‘PAGE_KERNEL_NOCACHE’ undeclared (first use in this function)
 arch/x86/kernel/early_printk.c:827: error: (Each undeclared identifier is reported only once
 arch/x86/kernel/early_printk.c:827: error: for each function it appears in.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>