Add the new gravity cache file to the 'make dist' target.

src/Makefile.am

@@ -64,7 +64,7 @@ nobase_noinst_HEADERS = align.h approx_math.h atomic.h cycle.h error.h inline.h
 		 kernel_long_gravity.h vector.h cache.h runner_doiact.h runner_doiact_vec.h runner_doiact_grav.h runner_doiact_fft.h \
                  runner_doiact_nosort.h units.h intrinsics.h minmax.h kick.h timestep.h drift.h adiabatic_index.h io_properties.h \
 		 dimension.h equation_of_state.h part_type.h periodic.h \
-		 gravity.h gravity_io.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  \
 		 sourceterms.h \