s_xor function to c11 standard

[shanginn]

this change is neccecery because of new c11 standard. otherwise when there will be compile error:

In file included from heavens_door.c:27:0:
hacking.h: In function ‘s_xor’:
hacking.h:166:2: error: ‘for’ loop initial declarations are only allowed in C99 mode
  for(int i = 0; i < nbytes; i++) arg[i] ^= key;
  ^
hacking.h:166:2: note: use option -std=c99 or -std=gnu99 to compile your code

(gcc 4.8.5 20150623 (Red Hat 4.8.5-16))

[f0rb1dd3n]

Thank you to report that. I am using c99 standard and I didnt see that.