Silence a valgrind false positive in the gravity code.

src/runner_doiact_grav.c

@@ -343,6 +343,7 @@ static INLINE void runner_dopair_grav_pp_full_no_cache(
     cache_i->a_y[i] += a_y;
     cache_i->a_z[i] += a_z;
     cache_i->pot[i] += pot;
+    cache_i->active[i] = 1;
   }
 
   /* Write back to the particle data */
@@ -546,6 +547,7 @@ static INLINE void runner_dopair_grav_pp_truncated_no_cache(
     cache_i->a_y[i] += a_y;
     cache_i->a_z[i] += a_z;
     cache_i->pot[i] += pot;
+    cache_i->active[i] = 1;
   }
 
   /* Write back to the particle data */