x86, microcode support: fix build error

fix:

  arch/x86/kernel/microcode.c:412: error: static declaration of ‘microcode_init’ follows non-static declaration
  include/asm/microcode.h:1: error: previous declaration of ‘microcode_init’ was here
  arch/x86/kernel/microcode.c:454: error: static declaration of ‘microcode_exit’ follows non-static declaration
  include/asm/microcode.h:2: error: previous declaration of ‘microcode_exit’ was here

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