fix "-Werror" build against latest libosmo-abis

libosmo-abisrecently marked the 'out_cb' of the subchan_demux
as 'const', which caused compiler warnings/errors.

Related: libosmo-abis.git Ia082b9fddf03d02afd007825a1588a3ef0dbedae
Change-Id: I0cf430980e50fa8094f0efc7642004fb5923c4c6
diff --git a/src/flip_bits.h b/src/flip_bits.h
index 319a181..249e45b 100644
--- a/src/flip_bits.h
+++ b/src/flip_bits.h
@@ -1,4 +1,4 @@
 #pragma once
 
 void init_flip_bits(void);
-uint8_t *flip_buf_bits(uint8_t * buf , int len);
+uint8_t *flip_buf_bits(uint8_t *out, const uint8_t *in, int len);