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: NFeng Tang <feng.tang@intel.com>
Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1398327498-13163-1-git-send-email-feng.tang@intel.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/include/asm/hpet.h

@@ -63,6 +63,7 @@
 /* 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

@@ -88,7 +88,7 @@ static inline void hpet_clear_mapping(void)
 /*
  * HPET command line enable / disable
  */
-static int boot_hpet_disable;
+int boot_hpet_disable;
 int hpet_force_user;
 static int hpet_verbose;