core: Add timespec helper macros and make timer_compat.h public

If a monotonic clock must be used, then the clock_gettime API is used
which uses timespec structures. Linux systems by default don't provide
helpers to calculate time using timespecs, so let's add them here.

Let's also make this header public so these helpers can be used in other
projects using libosmocore (expected user: libosmo-netif).

Change-Id: I45fc993b9bb0a343763238bf463c8640f47b00f1
diff --git a/include/Makefile.am b/include/Makefile.am
index f95d90c..8b05f80 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -47,6 +47,7 @@
                        osmocom/core/strrb.h \
                        osmocom/core/talloc.h \
                        osmocom/core/timer.h \
+                       osmocom/core/timer_compat.h \
                        osmocom/core/utils.h \
                        osmocom/core/write_queue.h \
                        osmocom/crypt/auth.h \
@@ -148,7 +149,6 @@
 endif
 
 noinst_HEADERS = \
-	osmocom/core/timer_compat.h \
 	osmocom/gsm/kasumi.h osmocom/gsm/gea.h
 
 osmocom/core/bit%gen.h: osmocom/core/bitXXgen.h.tpl