timer: Add compatibility header with timer helpers

Some of these are not always present, especially when cross compiling

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/src/timer.c b/src/timer.c
index da16459..217f652 100644
--- a/src/timer.c
+++ b/src/timer.c
@@ -38,6 +38,7 @@
 #include <string.h>
 #include <limits.h>
 #include <osmocom/core/timer.h>
+#include <osmocom/core/timer_compat.h>
 #include <osmocom/core/linuxlist.h>
 
 static struct rb_root timer_root = RB_ROOT;