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/include/osmocom/core/Makefile.am b/include/osmocom/core/Makefile.am
index 5465d5f..1df111a 100644
--- a/include/osmocom/core/Makefile.am
+++ b/include/osmocom/core/Makefile.am
@@ -6,6 +6,8 @@
 		   backtrace.h conv.h application.h \
 		   crcgen.h crc8gen.h crc16gen.h crc32gen.h crc64gen.h
 
+noinst_HEADERS = timer_compat.h
+
 if ENABLE_PLUGIN
 osmocore_HEADERS += plugin.h
 endif