x86: pgtable_32.h - prototype and section mismatch fixes

This patch adds extern to native_pagetable_setup_[start,done]() protypes and
fixes following section mismatch warning:

WARNING: arch/x86/mm/built-in.o(.text+0xf2): Section mismatch in reference from
the function paravirt_pagetable_setup_start()

paravirt_pagetable_setup_[start,done]() is used by __init pagetable_init().
Annotate both functions with __init.

Signed-off-by: Jacek Luczak <luczak.jacek@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>