timer_gettimeofday.c: Fix implicit use of timeradd

timer_gettimeofday.c: In function 'osmo_gettimeofday_override_add':
timer_gettimeofday.c:54:2: error: implicit declaration of function 'timeradd' [-Werror=implicit-function-declaration]
  timeradd(&osmo_gettimeofday_override_time, &val,
  ^

Change-Id: Iced451be255cfde8a6cf38380bef71ef29673994
diff --git a/src/timer_gettimeofday.c b/src/timer_gettimeofday.c
index 81a1598..3573051 100644
--- a/src/timer_gettimeofday.c
+++ b/src/timer_gettimeofday.c
@@ -29,6 +29,7 @@
 
 #include <stdbool.h>
 #include <sys/time.h>
+#include <osmocom/core/timer_compat.h>
 
 bool osmo_gettimeofday_override = false;
 struct timeval osmo_gettimeofday_override_time = { 23, 424242 };