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 5c94e99..3ea33f4 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1103,7 +1103,7 @@
 
     ret = ret >> ( biL - 1 );
 
-    return ret;
+    return (unsigned) ret;
 }
 
 /*