fixes for gpu

Author: szymonlopaciuk

tests/test_capi.py

@@ -655,8 +655,8 @@ class Cells(xo.Struct):
     src = r"""
     #include "xobjects/headers/common.h"
 
-    int MAX_PARTICLES = 4;
-    int MAX_CELLS = 3;
+    static const int MAX_PARTICLES = 4;
+    static const int MAX_CELLS = 3;
 
     GPUKERN void loop_over(
         Cells cells,