Include codec.h file from codec C files

failure to do so would prevent the compiler from catching
inconsistencies between declaration and definition.

Change-Id: I88617c97adcff328292e501fd5d254eeae96a660
diff --git a/src/codec/gsm610.c b/src/codec/gsm610.c
index 3346ffc..c1eca0c 100644
--- a/src/codec/gsm610.c
+++ b/src/codec/gsm610.c
@@ -26,6 +26,7 @@
 
 #include <osmocom/core/bitvec.h>
 #include <osmocom/core/utils.h>
+#include <osmocom/codec/codec.h>
 
 /* GSM FR - subjective importance bit ordering */
 	/* This array encodes GSM 05.03 Table 2.