Update octree.h

Author: TimGoTheCreator

src/gravity/octree.h

@@ -10,6 +10,7 @@
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #pragma once
+#include "struct/particle.h"
 #include "floatdef.h"
 #include "dt/softening.h"
 #include <vector>