ct_lt_mpi_uint: cast the return value explicitely

The return value is always either one or zero and therefore there is no
risk of losing precision. Some compilers can't deduce this and complain.
diff --git a/library/bignum.c b/library/bignum.c
index f1cf808..f07f746 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -951,7 +951,7 @@
 
     ret = ret >> ( biL - 1 );
 
-    return ret;
+    return (unsigned) ret;
 }
 
 /*