x86/platform/uv: Include clocksource.h for clocksource_touch_watchdog()

x86/platform/uv: Include clocksource.h for clocksource_touch_watchdog()

Orabug: 25477822

This build failure triggers on 64-bit allmodconfig:

  arch/x86/platform/uv/uv_nmi.c:493:2: error: implicit declaration of function ‘clocksource_touch_watchdog’ [-Werror=implicit-function-declaration]

which is caused by recent changes exposing a missing clocksource.h include
in uv_nmi.c:

  cc1e24fdb064 x86/vdso: Remove pvclock fixmap machinery

this file got clocksource.h indirectly via fixmap.h - that stealth route
of header inclusion is now gone.

Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit d51953b0873358d13b189996e6976dfa12a9b59d)
Signed-off-by: Brian Maly <brian.maly@oracle.com>