Change infinite loop bug status in 1990/theorem

If one passes to the Fibonacci or theorem_bkp program 0 0 it enters an
infinite loop repeatedly printing 0. I think this is better to be just
documented and not fixed. Another condition where this occurred was
fixed but as I recall that might have affected usability and this one in
any event does not.

1990/theorem/.gitignore

@@ -1,3 +1,4 @@
+theorem
 fibonacci
 fibonacci.c
 mariano