bitvec: Introduce bitvec_get_nth_set_bit() function

This is particularly important for determining the ARFCN for cells
reported in 04.08 MEAS REP.
diff --git a/openbsc/include/openbsc/bitvec.h b/openbsc/include/openbsc/bitvec.h
index a365e2a..b35aebf 100644
--- a/openbsc/include/openbsc/bitvec.h
+++ b/openbsc/include/openbsc/bitvec.h
@@ -42,6 +42,9 @@
 /* check if the bit is 0 or 1 for a given position inside a bitvec */
 enum bit_value bitvec_get_bit_pos(struct bitvec *bv, unsigned int bitnr);
 
+/* get the Nth set bit inside the bit vector */
+unsigned int bitvec_get_nth_set_bit(struct bitvec *bv, unsigned int n);
+
 /* Set a bit at given position */
 int bitvec_set_bit_pos(struct bitvec *bv, unsigned int bitnum,
 			enum bit_value bit);