x86/hpet: Make boot_hpet_disable extern

HPET on some platform has accuracy problem. Making
"boot_hpet_disable" extern so that we can runtime disable
the HPET timer by using quirk to check the platform.

Signed-off-by: Feng Tang <[e-mail]>
Cc: Clemens Ladisch <[e-mail]>
Cc: John Stultz <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/include/asm/hpet.h

···
 /* hpet memory map physical address */
 extern unsigned long hpet_address;
 extern unsigned long force_hpet_address;
+extern int boot_hpet_disable;
 extern u8 hpet_blockid;
 extern int hpet_force_user;
 extern u8 hpet_msi_disable;

arch/x86/kernel/hpet.c

···
 /*
  * HPET command line enable / disable
  */
-static int boot_hpet_disable;
+int boot_hpet_disable;
 int hpet_force_user;
 static int hpet_verbose;