bitvec: add bitvec_find_first_bit_pos() from gsm/rxlev_stat.c

This patch adds bitvec_find_bit_pos() to bitvec.c where it really
belongs to. Before this patch used to be part of gsm/rxlev_stat.c
diff --git a/include/osmocom/core/bitvec.h b/include/osmocom/core/bitvec.h
index 42977fb..bbe1641 100644
--- a/include/osmocom/core/bitvec.h
+++ b/include/osmocom/core/bitvec.h
@@ -68,6 +68,8 @@
 /* get multiple bits (based on numeric value) from current pos */
 int bitvec_get_uint(struct bitvec *bv, int num_bits);
 
+/* find the first bit set in bit vector */
+int bitvec_find_bit_pos(const struct bitvec *bv, unsigned int n, enum bit_value val);
 
 /* Pad the bit vector up to a certain bit position */
 int bitvec_spare_padding(struct bitvec *bv, unsigned int up_to_bit);