um: fix build without CONFIG_UML_TIME_TRAVEL_SUPPORT

When CONFIG_UML_TIME_TRAVEL_SUPPORT isn't set, the build was broken.
Fix this.

Fixes: 065038706f ("um: Support time travel mode")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/include/shared/timer-internal.h

@@ -54,6 +54,8 @@ static inline void time_travel_set_timer(enum time_travel_timer_mode mode,
 					 unsigned long long expiry)
 {
 }
+
+#define time_travel_timer_mode TT_TMR_DISABLED
 #endif
 
 #endif