pcu: Add bitvec_write_field_lh

While the bitvec functions from libosmocore support LH encoding, the
C++ wrapper does not.

Add a bitvec_write_field_lh function that is similar to
bitvec_write_field, but writes L and H instead of ZERO and ONE.

Sponsored-by: On-Waves ehf
diff --git a/src/bitvector.h b/src/bitvector.h
index 36bdbab..b14d204 100644
--- a/src/bitvector.h
+++ b/src/bitvector.h
@@ -39,6 +39,7 @@
 unsigned int bitvec_unpack(struct bitvec *bv, uint8_t *buffer);
 uint64_t bitvec_read_field(struct bitvec *bv, unsigned& read_index, unsigned len);
 int bitvec_write_field(struct bitvec *bv, unsigned& write_index, uint64_t val, unsigned len);
+int bitvec_write_field_lh(struct bitvec *bv, unsigned& write_index, uint64_t val, unsigned len);
 
 /*! }@ */