HV:header:fix "expression is not Boolean"

MISRA C explicit required expression should be boolean when
in branch statements (if,while...).

Signed-off-by: Huihuang Shi <huihuang.shi@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

hypervisor/include/arch/x86/timer.h

@@ -38,7 +38,7 @@ static inline void initialize_timer(struct timer *timer,
 				int mode,
 				uint64_t period_in_cycle)
 {
-	if (timer) {
+	if (timer != NULL) {
 		timer->func = func;
 		timer->priv_data = priv_data;
 		timer->fire_tsc = fire_tsc;