It's not in the "k.h" header, though that references it also.

Maybe the build system injects it through the compiler command line or a forced-include header. Though the gcc command line alluded to in the block comment header shows no evidence of that.

Edit: Found it! It's a typedef for a pointer to a struct k0:

  typedef struct k0{signed char m,a,t;C u;I r;union{G g;H h;I i;J j;E e;F f;S s;struct k0*k;struct{J n;G G0[1];};};}*K;