Added the new file to the Makefile.

src/Makefile.am

@@ -96,7 +96,7 @@ nobase_noinst_HEADERS = align.h approx_math.h atomic.h barrier.h cycle.h error.h
                  runner_doiact_functions_hydro.h runner_doiact_functions_stars.h runner_doiact_functions_black_holes.h \
 		 units.h intrinsics.h minmax.h \
                  kick.h timestep.h drift.h adiabatic_index.h io_properties.h dimension.h part_type.h periodic.h memswap.h \
-                 dump.h logger.h sign.h logger_io.h timestep_limiter.h hashmap.h \
+                 dump.h logger.h sign.h logger_io.h timestep_limiter.h timestep_sync.h hashmap.h \
 		 gravity.h gravity_io.h gravity_cache.h \
 		 gravity/Default/gravity.h gravity/Default/gravity_iact.h gravity/Default/gravity_io.h \
 		 gravity/Default/gravity_debug.h gravity/Default/gravity_part.h  \