Added missing file to the master Makefil.am

src/Makefile.am

@@ -116,7 +116,7 @@ AM_SOURCES = space.c runner_main.c runner_doiact_hydro.c runner_doiact_limiter.c
 
 # Include files for distribution, not installation.
 nobase_noinst_HEADERS = align.h approx_math.h atomic.h barrier.h cycle.h error.h inline.h kernel_hydro.h kernel_gravity.h \
-		 gravity_iact.h kernel_long_gravity.h vector.h accumulate.h cache.h \
+		 gravity_iact.h kernel_long_gravity.h vector.h accumulate.h cache.h exp.h \
 	         runner_doiact_nosort.h runner_doiact_hydro.h runner_doiact_stars.h runner_doiact_black_holes.h runner_doiact_grav.h \
                  runner_doiact_functions_hydro.h runner_doiact_functions_stars.h runner_doiact_functions_black_holes.h \
 		 runner_doiact_functions_limiter.h runner_doiact_limiter.h units.h intrinsics.h minmax.h \