Add allow_backprop member of McXtrace photon struct

Author: willend

mccode/src/cogen.c.in

@@ -2448,11 +2448,11 @@ cogen_header(struct instr_def *instr, char *output_name)
   cout("  double sx,sy,sz; /* spin [0-1] */");
   cout("  int mcgravitation; /* gravity-state */");
   cout("  void *mcMagnet;    /* precession-state */");
-  cout("  int allow_backprop; /* allow backprop */");
 #elif MCCODE_PROJECT == 2 /* xray */
   cout("  double kx,ky,kz; /* wave-vector */");
   cout("  double phi, Ex,Ey,Ez; /* phase and electrical field */");
 #endif
+  cout("  int allow_backprop; /* allow backprop */");
   cout("  /* Generic Temporaries: */");
   cout("  /* May be used internally by components e.g. for special */");
   cout("  /* return-values from functions used in trace, thusreturned via */");